Nelson Mandela accused President Mugabe of a “tragic failure of leadership” last night, as southern Africa turned its back on the Zimbabwean leader.

Mr Mandela spoke of his concern and sadness at the chaos engulfing Zimbabwe, amid clear indications that the patience of Mr Mugabe’s remaining allies was at breaking point.

Wielding the moral authority of the world’s best-known statesman, Mr Mandela broke his silence at a fundraising event to mark his 90th birthday celebrations in London.

Hours before he spoke, Zimbabwe’s neighbours presented a united front for the first time and urged Mr Mugabe to call off Friday’s presidential vote.

The leadership of the Southern African Development Community (SADC) said that his re-election in a one-horse race would lack any credibility.

Mr Mugabe was also stripped of his honorary knighthood by the Queen, on the advice of the Government. And Gordon Brown told British companies not to “prop up” his regime. After yesterday’s disclosure in The Times that Anglo American was investing millions in a platinum mine project, the company said that it was “reviewing all options surrounding the development”.

Mr Mandela, South Africa’s former President, decided to speak out after months of keeping his counsel only after he arrived in Britain on Monday to learn that Morgan Tsvangirai had pulled out of Friday’s election.

Mr Tsvangirai, the Zimbabwean opposition leader, said that it was impossible to proceed with the vote because of the murder, torture and harassment of his supporters.

Mr Mandela’s intervention, at an event attended by Gordon Brown and Bill Clinton, will pile the pressure on President Mbeki, his successor. Mr Mbeki has avoided public criticism of Mr Mugabe, but may now feel obliged to speak out.

Ministers insisted that the stripping of Mr Mugabe’s honorary knighthood should not be seen as a key part of the efforts to tackle the crisis in Zimbabwe. They had been wary of removing it for fear of giving Mr Mugabe a propaganda tool but David Miliband, the Foreign Secretary, decided this week that the move was appropriate given international revulsion at the regime.

An emergency summit of southern African leaders called yesterday for Zimbabwe’s run-off presidential election to be postponed because of the recent violence.

The leaders of Swaziland, Tanzania and Angola, the troika of countries representing the SADC, said that the conditions would not permit a free and fair election. Although some of Zimbabwe’s neighbours had spoken out previously, it was the first time that the leadership of the SADC, which is responsible for overseeing peace and security in southern Africa, had taken a collective stance.

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