Ramphele had agreed to seek the presidency for the opposition Democratic Alliance, which is trying to throw off perceptions it represents the interests of the white minority, against the ruling African National Congress (ANC).
Her change of heart frustrates the party’s drive to woo black voters from President Jacob Zuma’s ANC, which has been in power since the end of white-minority rule in 1994.
“The time for this was not right,” she told a news conference, saying she had rushed into the decision to join the Democratic Alliance and would stay with her own Agang party.
Agang has gained little traction in its one-year existence despite growing voter disaffection with the ANC.
Ramphele’s defection to the Democratic Alliance had led some to accuse the physician and author of opportunism, and had threatened her standing among the black majority as the partner of Biko, who was beaten to death by apartheid police in 1977.
Democratic Alliance leader Helen Zille denounced her old friend for reneging on the election deal so swiftly. “Dr Ramphele has demonstrated once and for all that she cannot be trusted to see any project through to its conclusion,” she said.
The ANC’s support has waned amid corruption scandals and charges it has failed to rescue millions of blacks from poverty, but it won nearly two-thirds of the vote in the last election in 2009 and its overall majority this year is not in question.
No date has been set for the parliamentary polls, which are expected to be held by May.