Condemning Tony Blair for his robust support of American threats to disarm Iraq by force with or without UN approval, Mr Mandela said that the Prime Minister had become “the foreign minister of the United States. He is no longer the Prime Minister of Britain.”

Mr Mandela asked: “Why is the United States behaving so arrogantly?” Saying that all America wanted was Iraqi oil, he accused President Bush and Mr Blair of undermining the authority of the United Nations because Kofi Annan, the Secretary-General, was black.

“Is it because the Secretary-General of the United Nations is now a black man?” Mr Mandela said. “They never did that when secretary-generals were white.”

He called for the American people to rise up in protest against President Bush and urged all world leaders with vetoes in the United Nations Security Council to unite in opposition to the US-British plans for disarming Iraq.

“One power with a President who has no foresight and cannot think properly is now wanting to plunge the world into a holocaust,” Mr Mandela, 84, told a cheering audience at an international women’s conference in Johannesburg.

In recent months, Mr Mandela, a Nobel Peace Prize laureate, has repeatedly criticised the policy of President Bush and Mr Blair towards Iraq, demanding that Washington and London respect the UN’s authority. But yesterday’s speech was far harsher and more personal than anything hitherto.

Mr Mandela said that the United Nations was the only reason that there had not been a third world war, and that it should be up to the Security Council alone to decide how to deal with the regime of President Saddam Hussein.

The United States, which had callously dropped atomic bombs on Hiroshima and Nagasaki, had no moral authority to police the world, Mr Mandela said. “If there is a country that has committed unspeakable atrocities in the world, it is the United States of America. They don’t care for human beings.”

Mr Blair and Thabo Mbeki, the South African President, are to meet at Chequers tomorrow. Mr Mbeki is expected to give dire warnings about the consequences of a war against Iraq for Africa’s prospects for economic recovery.

Mr Mbeki, who will also be speaking for the African Union and the Non-Aligned Movement, is expected to exhort Mr Blair to pull back from using force to disarm Baghdad, and to grant UN weapons inspectors more time.

Fearing the effects of a 1970s-style oil-shock, in which prices could reach $80 (£48) a barrel, Mr Mbeki will tell Mr Blair that such a development would effectively mean saying goodbye to African economic progress.

The four to five hours of largely one-to-one talks will also cover Zimbabwe, the war against terrorism, the Middle East and other regional issues such as the New Partnership for Africa’s Development (Nepad), although there is no formal agenda, British diplomats said.

Mr Mbeki, who has said that Iraq has not hindered the UN inspectors, earlier called on all South Africans to join the world peace movement to help to prevent a US-led war against Iraq.

He has also offered the services of R.F. “Pik” Botha, the former Foreign Minister, who played a key role in co-operating with the International Atomic Energy Agency over the destruction in 1993 of the seven atomic weapons built by apartheid South Africa, in helping Iraq “to improve its co-operation” with UN weapons inspectors.

Defending Pretoria’s “no war at any cost” position, Mr Mbeki said that it took UN inspectors two years to verify South Africa’s disarmament. “It is clear to us that it is necessary to give additional time to the inspectors,” he said.

“We are united in our conviction that weapons of mass destruction must be liquidated, but there is no need to go to war to do this. War would create new and enormous problems. This is not necessary. Nothing has happened to suggest that the Security Council must take a route that leads to war,” Mr Mbeki added. Mr Blair is likely to give Mr Mbeki a cordial hearing. But Mr Mbeki has already stated publicly that he has no power to alter the course of events.

Although South Africa’s ruling African National Congress cultivated close ties with the British Labour Party during its years in exile, relations between the two since Mr Blair came to power in 1997 have undergone periods of severe strain.

Britain remains by far the largest foreign investor in South Africa and is also its third largest trading partner. But Pretoria’s post-apartheid foreign policy has been preoccupied with developing new ties with Africa and the rest of the Third World rather than with the West.

Britain has denounced President Mugabe’s illegal land seizures in Zimbabwe, the destruction of the rule of law and last year’s fraudulent presidential election. But South Africa has consistently refused to condemn Mr Mugabe’s regime, saying that Africans should be left alone to sort out Africa’s problems.

South Africa also recently found itself the target of intense Western criticism after voting for Libya, a former sponsor of terrorism with an appalling record on human rights, to take the chair of the United Nations Human Rights Commission.

Mr Blair, initially an enthusiastic supporter of Nepad, in which the West promised billions of pounds in aid and investment in exchange for a commitment to uphold democracy, the rule of law and good governance, is now calling for African leaders keep their part of the bargain.

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