The former South African president, who won the Nobel Peace Prize in 1993, has always denied being a member of the South African branch of the movement, which mounted an armed campaign of guerrilla resistance along with the ANC.
But research by a British historian, Professor Stephen Ellis, has unearthed fresh evidence that during his early years as an activist, Mr Mandela did hold senior rank in the South African Communist Party, or SACP. He says Mr Mandela joined the SACP to enlist the help of the Communist superpowers for the ANC's campaign of armed resistance to white rule.