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

Last week's agreement would have merged the DA with the Agang party of anti-apartheid activist Mamphela Ramphele.

Ms Ramphele had been put forward as the DA's presidential candidate for this year's elections.

The merged party had been billed as the strongest challenger to the ANC since it came to power in 1994.

Ms Ramphele was the partner of anti-apartheid campaigner Steve Biko, who died in police custody in 1977.

The short-lived union had been billed as a historic change to South Africa's political landscape, as it would have been the first time the DA had fielded a black presidential candidate.

Analysts say it has failed to make significant headway among the majority black electorate because it was seen as "too white".

DA supporters had hoped Ms Ramphele would have helped the party tap into voter dissatisfaction with President Jacob Zuma and the ANC, under fire over corruption scandals and stubbornly high poverty levels.

"Dr Ramphele reneged on the agreement that she stand as the DA's presidential candidate, and that Agang SA's branches, members and volunteers be incorporated into the DA," said DA leader Helen Zille, in a statement.

She angrily accused Ms Ramphele of saying one thing to the media, another to her Agang supporters and yet another to the DA.

"Dr Ramphele has demonstrated - once and for all - that she cannot be trusted to see any project through to its conclusion," she said.

But Ms Ramphele, responding via Twitter, insisted: "There was no confusion. I remain committed and still am the leader of Agang SA. The focus was to work together in the election."

On Friday, the DA published a joint statement announcing that Ms Ramphele would join its ranks. DA rules say only its members can serve as candidates under its banner.

Ms Ramphele said the statement had been published without her agreement and that she would remain the leader of Agang SA.