South Africa's main opposition political party has announced the collapse of a plan to join forces with another opposition group to challenge the ruling party in elections this year.

The Democratic Alliance party said in a statement on Sunday that Mamphela Ramphele, the opposition leader, had reneged on a deal to be its presidential candidate and to merge her smaller Agang party with the Democratic Alliance.

"Dr Ramphele has demonstrated, once and for all, that she cannot be trusted to see any project through to its conclusion. This is a great pity,'' Helen Zille, the Democratic Alliance leader, said in the statement.

Her party promised to brief journalists on the coalition breakdown on Monday.

Only five days earlier, the two parties held an upbeat news conference to announce  a merger.

The sudden turnaround reflected the deep divisions within opposition ranks in South Africa, which has been led by the African National Congress, the chief liberation movement during white racist rule, since Nelson Mandela was elected president in South Africa's first all-race elections in 1994.

Analysts expect the ruling party to win this year's elections, though possibly with a smaller majority.

Ramphele was the partner of Steve Biko, the Black Consciousness leader who was tortured and died in police custody in 1977. She has been an activist, doctor, academic and World Bank executive.

"It is not clear what her objective is, but whatever it is, it is not in the interests of the South African people,'' said Zille, who said Ramphele had insisted on going public last week with her acceptance of the offer to be presidential candidate for the Democratic Alliance.

Zille described Ramphele as a "longtime personal friend of mine" and said she had sought to bring Ramphele into politics over many years.

Ramphele last year formed her own party, named Agang, or Build, in the Sesotho language, but struggled to gain political momentum.

In a statement on Friday, she said she remained leader of her party and that claims that she would be accepting membership in the Democratic Alliance were false.