Allan Boesak was regarded as a hero during the fight against apartheid
South Africa's prominent anti-apartheid activist Allan Boesak has resigned from the new Congress of the People, citing squabbles within the opposition party.
Cope was formed by dissidents from the ruling ANC last year angered by the sacking of Thabo Mbeki as president.
Mr Boesak said the party was in "disarray" due to "faction-fighting and battles for political supremacy".
Cope spokesman Phillip Dexter told the BBC his resignation came as a surprise and denied allegations of party rifts.
The African National Congress welcomed Mr Boesak's decision and invited him to re-join its ranks.
Cope won 7.42% of the vote in April's elections which saw ANC leader Jacob Zuma become president.
The party was formed ahead of the polls after a power struggle between Mr Zuma and Mr Mbeki.
"From the very beginning the party structures, such as they were, were characterised by faction-fighting, strife, pitched battles for political supremacy," said Mr Boesak, who won a seat in Western Cape's provincial parliament in the election.
He said there was "deep resentment" within the party which made "normal work almost impossible".
Mr Boesak's announcement follows the resignation of two other leading Cope members, Simon Grindrod and Linda Odendaal, in recent months.
"We were not expecting him to resign," Mr Dexter told the BBC's Focus on Africa programme.
"He had expressed his view and some of his unhappiness, but these were challenges we were dealing with."
Mr Dexter said Cope's profile was improving and the party was generally "doing better".
Mr Boesak played a pivotal role in the anti-apartheid coalition the United Democratic Front in the 1980s when many liberation movements, including the ANC, were banned in South Africa.
His career was tainted by corruption in the early 1990s, but he managed to rejuvenate his career and the announcement that he was joining Cope was greeted with rapturous applause last year at the party's official launch.
Western Cape is the only province that is not controlled by the ANC.