"Dr Ramphele has decided to retire as chair to further her socio-economic and political work," spokesman Sven Lunsche said in statement.

"She has resigned from most of her private sector directorships and will relinquish all other remaining private sector directorships and positions to focus on these activities."

He said non-executive director Cheryl Carolus would succeed Ramphele.

The move intensified speculation she is about to launch a political party to challenge the ruling African National Congress (ANC).

Ramphele, 65, commands considerable respect among South Africa’s black majority as a partner of Black Consciousness leader Steve Biko, who died in 1977 as a result of beatings received in an apartheid prison.

She was also placed under house arrest for seven years by South Africa’s white-minority government because of her political activities.

“She is an amazingly powerful woman,” said political analyst Nic Borain, adding that Ramphele would not be taking a leap into direct politics against the might of Nelson Mandela’s 100-year-old liberation movement without having a solid plan in place.

“She would have done her homework,” he said.

Another political analyst, Allister Sparks, said a Ramphele-led party would be likely to take votes from the ANC, which has ruled virtually unchallenged since the end of apartheid in 1994, rather than the main opposition Democratic Alliance (DA), still seen as the party of white privilege.

“She is not just a fine academic; she is a dynamic woman of action,” he wrote in an editorial in Wednesday’s Business Day newspaper.

A consultancy working for Ramphele said she would be making a statement about her political plans on Johannesburg’s Constitution Hill on Monday.