A deal which would have given South Africa's main opposition Democratic Alliance (DA) a black presidential candidate collapsed .

Last week's agreement would have merged the Alliance with the Agang party of anti-apartheid activist Mamphela Ramphele, who had been put forward as the DA presidential candidate.

But she will not now run as the alliance's presidential candidate in this year's election, opposition party leader Helen Zille confirmed .

Ms Zille said that Dr Ramphele had changed her mind after agreeing last week to join the opposition party.

"By going back on the deal again, just five days after it was announced, Dr Ramphele has demonstrated once and for all that she cannot be trusted to see any project through to its conclusion," said Ms Zille bitterly.

But Dr Ramphele - a medical doctor and former World Bank managing director - said that she had rushed the decision to join the DA.

"The time for this was not right," she admitted.

Dr Ramphele's Agang party has struggled to gain traction against President Jacob Zuma's ANC, which has held power since the end of white-minority rule in 1994.

Alliance supporters had hoped Dr Ramphele would help it exploit voter dissatisfaction with the ANC.

But the deal fell apart at a meeting on Sunday.

"Dr Ramphele reneged on the agreement that she stand as the DA presidential candidate and that Agang branches, members and volunteers be incorporated into the DA," said Ms Zille.

She angrily accused Dr Ramphele of saying one thing to the media, another to Agang supporters and yet another to the alliance.

On Friday the alliance published a joint statement announcing that Dr Ramphele would join it, but Dr Ramphele said the statement had been published without her agreement and she would remain the leader of Agang.

However, Agang is now bankrupt and a month ago was unable to pay its staff.

The ANC had dismissed the pact with the alliance.

"It's a rent-a-leader and rent-a-black face," ANC general secretary Gwede Mantashe had observed sharply.