Anthony Sampson, who has known Nelson Mandela for 50 years, pays a birthday tribute to the statesman who wants a quiet life but is still drawn to the public stage, the world icon who, in his old age, has grown angrier and more outspoken than ever.

Last Wednesday, Nelson Mandela once again showed his unique moral influence, two weeks before he celebrates his eighty-fifth birthday with a banquet in Johannesburg. In Westminster Hall, he launched the Mandela-Rhodes Foundation which will bring part of the huge fortune built on diamonds and gold back to black South Africa. He heard tributes from Bill Clinton and Tony Blair, with whom he had talked at length at Number 10 beforehand.

The Prime Minister made an impromptu speech explaining how Mandela 'symbolised the triumph of hope over injustice'. Mandela warmly thanked him but did not conceal their differences about the Middle East: 'We differ on one point - very strongly,' he said.

Afterwards, Blair and Clinton supported the frail old man as he walked slowly down the hall between the audience of a thousand Rhodes Scholars who were clapping Mandela much more than them. It was a poignant image: the two much younger leaders still needed the moral support of the old man.

How has a man with no formal political position retained such influence? Since Mandela retired as President of South Africa four years ago, when I wrote his authorised biography, I have talked to him often in London and South Africa, and have always been surprised. Since I first met him 50 years ago in Johannesburg, I have seen him in many different roles -- as lawyer, revolutionary, prisoner, electioneer, President and global icon.

At each stage, many people have expected a let-down or, at least, an anti-climax. But in retirement, he provides many new surprises. He has become a more outspoken and, sometimes, angry old man, protesting against injustices -- and now most notably against the warlike attitudes of Britain and America in the Middle East. It's a confrontation which is now coming to a head.

Yet, when he first retired, Nelson Mandela talked much about spending a quiet time with his family in his new house in his tribal village of Qunu, the poor but beautiful area of Transkei. He was content, he claimed, to leave the future of his country to his successor, President Mbeki. He no longer wanted to be known as Mr President, but as 'Madiba', his clan-name.

Now, he sometimes still likes to pretend to be a forgotten figure, 'an unemployed pensioner', as he describes himself. He retells the story of a young girl who told him he was a silly old man who broke the law and went to jail. When I reintroduced my wife to him saying: 'You remember Sally?' he replied: 'Yes -- but do you remember me?'

Of course he knows perfectly well that everyone remembers him. (Meanwhile, his own memory remains prodigious into old age; like several of his long-term prison colleagues, his memory appears to have been improved by his time in jail, away from the distractions and soundbites of television and advertising.) The name Mandela is attached to streets, squares, scholarships and buildings across the world, and an elegant new bridge across central Johannesburg, which will be opened on his birthday.

The manner of his retirement in 1999 is in itself a tribute to his achievement of a new South Africa. Five years earlier, before his own inauguration, most South Africans doubted whether elections could be held at all, in the face of violent threats by his opponents to boycott them. Now, South Africans of all colours take for granted that their country is a multiracial working democracy. This is Mandela's most valuable legacy.

At his own inauguration, the new President, Thabo Mbeki, made a short, unpompous speech warning about threats elsewhere, including dangers from 'our own African predators', but he was confident about South Africa's future with a place for all races and a 'common destiny regardless of the shapes of our noses'.

Mandela gave a short farewell speech beginning with an unscripted joke. 'I made a mistake by being ejected from the presidency. Next time, I will choose a Cabinet which will allow me to be life President.' But it was more than a joke: he was certainly losing power with some regrets, but he was reminding his audience how many democratically elected leaders in Africa, including Mugabe in Zimbabwe, had turned themselves into dictators. He added a postscript: 'I must find Madiba a job, because old men get up to mischief.' That, too, has proved not entirely flippant.

For the first time since he left prison nine years before, Mandela became in 1999 a private individual without any political position. By then he had been happily married for a year to his third wife, Graca, the widow of President Machel of Mozambique, and enjoying his grandchildren and great-grandchildren, moving between his house in Johannesburg, his country retreat at Qunu and Graca's mansion in Mozambique. Graca is a strong character, a politician in her own right, and Mandela likes to pretend that she dominates him: 'At least she allowed me to keep my surname.'

But she has provided him with the kind of support he did not enjoy from his second wife, Winnie: 'I want him as a human being,' Graca told me. 'He is a symbol, but not a saint. Whatever happens to him, it is a mark of the liberation of the African people.'

There is a constant paradox within his character. While he claims he wants a quiet life, he is also still drawn to the public stage. While he wants to give other people the chance to take the spotlight, he remains one of the most famous men alive. He's still driven to have more experiences, to catch up on his 26 lost years in prison. He's become more, not less, impatient, an old man in a hurry.

'I have retired,' he said at 84, 'but if there's anything that would kill me it is to wake up in the morning not knowing what to do.' Graca tried to make him relax, but she soon changed her mind. 'I realised I was making a mistake,' she told me. 'He needs to be very busy. He is quite clear that if he slows down he will feel depressed. He'll feel he is not needed any more.'

In 1999, he established the Nelson Mandela Foundation, which has provided his base. His loyal Afrikaner secretary, Zelda le Grange, who comes from a very conservative nationalist family, organises his endless meetings, flights and phone calls to the world's leaders. He keeps crossing the world, particularly to Britain, America and the Middle East, often in a private plane provided by one of his rich friends, such as Prince Bandar, the Saudi ambassador in Washington.

He has embarked on writing the second volume of his memoirs, covering his presidential years, with help from his old colleagues. He's determined to write them himself, without being ghosted, and he conducts his research with very personal methods: he rings up old friends and even former enemies, such as ex-President de Klerk, to ask for their recollections of crucial meetings.

But he also loves meeting showbusiness heroes, sportsmen and film stars. In his last years in prison, he enjoyed watching Hollywood films, and he's still thrilled to meet real-life stars such as Whoopi Goldberg or Whitney Houston, whom he welcomed with outrageous flattery ('I'm only here to shine her shoes').

His more solemn colleagues complain that he's a sucker for famous faces who have no political importance but, in my view, he's merely radiating a wider love of life, beyond middle-aged men in suits. I often feel he's reliving his own youth in Johannesburg in the 1950s, when he was not only a politician, but a township hero, ladies' man, dancer and boxer.

Whenever he introduces me to others, he says: 'I first met Tony in a shebeen.' He still loves talking about the old black musicians and Drum writers, and his boxing days. Talking to him in London two years ago, we were interrupted to be told that another visitor was waiting, the boxer Frank Bruno. Mandela wasted no time breaking off our interview.

However, he's more lonely now. As President, he was at least 30 years older than most of the politicians in South Africa, and most of his contemporaries are dead. He often looks his age. Away from the cameras, he can suddenly look exhausted and sad; with his staff, he can be irritable. He sometimes loses his place in written speeches, though he makes a joke of it: 'Unlike some politicians, I can admit to a mistake.' But he retains a powerful will to live. In 2001, he was diagnosed with cancer of the prostate, but after intensive treatment, he soon appeared fully recovered. 'Very often the doctors are wrong,' he assured me later. 'But if cancer gets the upper hand, I will nevertheless be the winner. In heaven, I will be looking for the nearest branch of the ANC.'

Mandela often reflects on his past career, sometimes with remorse: he remembers his earlier years as a struggling and quite arrogant young politician and feels guilty about neglecting friends who have helped him on his way up. He worries that his political colleagues are being forgotten while he's so much honoured. He was shocked that university students had not heard of Oliver Tambo, his close friend and law partner, who had been the President of the ANC in exile while he was in prison, and who died in 1993.

He went out of his way to praise Walter Sisulu, his first mentor who had transformed the ANC in the Fifties, and who died in May. 'He has not occupied any great position,' Mandela said at the time of Sisulu's ninetieth birthday in 2002, 'but he stands head and shoulders above any of us, because he had two qualities: humility and simplicity -- and steel in his soul.' When Sisulu died, Mandela acknowledged his most crucial influence. 'By ancestry, I was born to rule,' he said, but Sisulu 'helped me to understand that my real vocation was to be a servant of the people.'

Could Mandela really have been content to take a back seat in South Africa? He warned his colleagues after he retired that he would feel free to criticise the leadership 'as an ordinary member of the ANC'. But he knows he is no ordinary member. He remains the most famous South African, pursued by cameras and journalists reporting anything controversial.

He is careful not to upstage or embarrass his successor, President Mbeki; he's more careful than Thatcher was with Major, or Churchill with Eden. He largely avoids commenting on domestic affairs and, when he travels abroad, he talks mainly about the need for reconciliation and peacemaking. But during his years as President, he had strained relations with Mbeki, who was his deputy. He emphasised to me that the choice of his successor was not made by him, but by the ANC and its allies. He was worried privately that Mbeki was too suspicious of his colleagues, too dependent on a few cronies.

And, in retirement, his relations have become more tricky. At public occasions together, Mandela inevitably overshadows Mbeki and often wins more applause. In public, Mandela has begun to make remarks which appear to go against Mbeki's policies, while in private ANC meetings he can be critical. 'I don't want to be a praise singer,' he explained after one closed conference. 'I want to be objective, and I did indicate his weaknesses, which was unpalatable to many members.'

Mbeki, in turn, has become more obviously resentful of the prominence given to his predecessor who looms above him, both politically and physically. Sometimes, he omits Mandela from state occasions; sometimes he is slow to return his phone calls. As President, Mandela was very careful to be punctual with visitors, determined to contradict the reputation of black leaders for keeping 'African time'. He sometimes resents Mbeki's less scrupulous attitude to timekeeping. When Mbeki arrived very late at a Mandela family wedding party, Mandela could not resist a thinly disguised reproof.

However, it is Mbeki's handling of the South African Aids crisis which has provoked the most obvious tensions. Mbeki was taking far too long to face up to the problem and to publicise the danger; Mandela was impatient for bolder action, but he himself was not blameless on the matter.

The danger was already very evident before Mandela became President in 1994, but he failed to raise the issue in his election campaign because, as he later admitted, it was not a popular issue. Many black South Africans are shy of condoms and contraception which question macho attitudes to sex.

When Mandela became President, he was preoccupied by the tasks of reconciliation and resisted calls to lead a major campaign against Aids. Edwin Cameron, a courageous gay South African judge who was found to be HIV-positive, became a prominent campaigner against Aids, and later reluctantly but firmly criticised Mandela's inaction.

'He, more than anyone else, could have reached into the minds and behaviour of young people,' said Cameron. 'A message from this man of saintlike, in some ways almost godlike, stature, would have been effective. He didn't do it. In 199 ways, he was our country's saviour. In the 200th way, he was not.'

In retirement, Mandela has been determined to break through the taboo. In August 2002, he made a deliberate political gesture when he publicly embraced a militant Aids activist, Zackie Achmat, who was HIV-positive, holding his arm over his shoulder, a gesture which was perpetuated in a photograph which was reproduced around the world. Many Africans have died of Aids without their families admitting the cause. But Mandela disclosed that three members of his own family had died of Aids. 'There is no shame,' he said, 'to disclose a terminal disease from which you are suffering.'

In Cape Town in February 2002, I watched Mandela making a moving appeal for action, endorsing a major initiative from the American Kaiser Foundation. It was just before President Mbeki made his annual speech to Parliament, which infuriated the government. Recently, Mandela has become more approving of Mbeki's campaign about Aids: after a private meeting a few months ago he told Mbeki he was quite satisfied.

These days, Mandela spends much of his time abroad, where his speeches are less controversial. He travels tirelessly, still making up for his long years of isolation, relishing his foreign friendships and grand receptions. In Britain and America, he is welcomed by conservatives as well as the Left, and he enjoys being entertained by rich businessmen who compete to honour him, even when he brings an entourage of aides and bodyguards. In London, where I see him often, he stays in a large suite in the Dorchester Hotel where the servants line up to greet him. He often calls on the Queen, with whom he enjoys a great friendship. He broke with protocol by writing to her as 'Dear Elizabeth'.

But he still seems happiest chatting with children in the street, who follow him as a fairytale figure, the prisoner who became a prince; a new post-apartheid generation still come under his spell, perpetuating his fame. When the BBC decided to devote two hours of primetime television to Mandela, it was at the instigation of the director-general, Greg Dyke, whose daughter had insisted that Mandela was her hero.

Since leaving jail in 1990, Mandela had felt very warmly towards America and Britain, but he has became more critical of their foreign policies, particularly after the Kosovo war. 'I am resentful about the type of thing that America and Britain are doing,' he told me in London in 2000. 'They want now to be the policemen of the world.'

As a lawyer, he sees Washington undermining the fragile basis of international law. 'They're introducing chaos into international affairs,' he said. 'Any country can take a decision which it wants. We have no lethal weapons such as the West. What they are doing is far more serious than what is happening in Africa.'

He suspected that the Americans were neglecting the UN partly because the secretary-general, Kofi Annan, came from Africa. 'From the time of Boutros Boutros-Galli, their attitude was totally different,' he told me. 'There are many people who are whispering that it's because the secretary-general is black.' Annan himself had made no such complaint, but Mandela has repeated the charge over the next three years.

He is much more worried about American domination after 11 September 2001. He immediately condemned the outrage. He talked with President Bush in Washington, and said that Osama bin Laden should be held responsible, captured and tried. But many of his closest political colleagues have been Muslims, who later persuaded him to modify his support for Bush, explaining that American policy could 'be seen as undermining some of the basic tenets of the rule of law'.

He has warned that the war against terrorism must not itself adopt the weapons of terror. And he is more strongly opposed to Israeli policies towards Palestinians, like many of his Jewish colleagues in the ANC -- including the outspoken Minister, Ronnie Kasrils -- who are now more critical of Israel than any other section of the Jewish diaspora.

He had enjoyed a close relationship with George Bush's father, who had been the first head of government to congratulate him on leaving jail. But he distrusts some of his closest advisers, whom his son has inherited, particularly Dick Cheney, who had voted in Congress against calling for Mandela's release from prison.

As the young Bush turned against Iraq, Mandela stepped up his warnings of the dangers of ignoring the UN, without success. When, in August 2002, he could not get through to Bush at his ranch in Texas, he called his father and asked him to talk to his son. In October, he gave an explosive interview to Newsweek which angrily recapitulated past American blunders in Iran and Afghanistan.

He accused President Bush of wanting to please the arms and oil companies. He described Bush's advisers, Dick Cheney and Donald Rumsfeld, as 'dinosaurs who do not want him to belong to the modern age'. And he attacked both America and Britain for racist attitudes. They did not criticise Israel for having weapons of mass destruction, he complained, because Israelis were seen as white, while Iraqis were seen as black.

As Bush and Blair prepared for war in Iraq, Mandela believed, as he told me, that neither was taking the UN seriously enough. He talked to Blair on the phone in December, reminding him that Churchill had supported the creation of the UN as the safeguard of world peace.

But he found Blair defensive and felt that he was closing ranks with Bush. Mandela finally exploded in late January in a speech to the International Women's Forum, abandoning a cautious text to speak - he told a colleague - 'from the heart'.

'It is a tragedy what is happening, what Bush is doing in Iraq,' he told his surprised audience. 'What I am condemning is that one power, with a President who has no foresight, who cannot think properly, who is now wanting to plunge the world into a holocaust.' And he went on to describe Tony Blair as Bush's 'Foreign Minister'.

Blair's officials in London, who were preparing for an important visit from President Mbeki, were shocked. But Mandela made some amends with another speech criticising Saddam, and Mbeki had a congenial time at Chequers the following weekend.

Mandela still hoped to mediate to prevent a war. But his links with Washington were weakening. He could not get through to Bush; Condoleezza Rice did not return his calls; even Colin Powell was apparently turning from dove to hawk.

He rang but failed to contact Saddam Hussein, who was refusing any foreign calls (Mandela was told) to avoid revealing his whereabouts. In February, Mandela even offered to fly to Iraq himself, provided he was asked by the UN.

When America and Britain finally went to war with Iraq, Mandela avoided further criticism. But in the last few days, he has returned to criticising President Bush and American foreign policy, just before Bush visits South Africa and other African countries this week. The White House remains puzzled by Mandela's outbursts: 'What does he want from us?' asked one aide.

But Mandela is still determined to represent his people's views and refuses to climb down. And he remains concerned about the mounting tension between Christians and Muslims. He is proud of the religious tolerance in his own government, which included several Muslim Ministers, and he believes that South Africa can help to bridge the religious divide in the rest of the world.

At 85, Mandela can still exercise his particular spell over the British and Americans. He is the fairytale figure, the prisoner who became President, who caught the imagination of crowds and children. But the myth is still connected to a statesman who is able to play a role in the dangerous and divided world after 11 September.

He can command equal respect on both sides, while his 85 years have given him experience of both power and powerlessness. He can speak for the huge populations in the developing world who are ignored by the superpowers, while he retains his moral authority in the West, even in America, as the champion of reconciliation and a multiracial society.