As evidence of Mr Mandela's Communist party membership, Prof Ellis cites minutes from a secret 1982 SACP meeting, discovered in a collection of private papers at the University of Cape Town, in which a veteran former party member, the late John Pule Motshabi, talks about how Mr Mandela was a party member some two decades before.
In the minutes, Mr Motshabi, is quoted as saying: "There was an accusation that we opposed allowing Nelson [Mandela] and Walter (Sisulu, a fellow activist) into the Family (a code word for the party) ... we were not informed because this was arising after the 1950 campaigns (a series of street protests). The recruitment of the two came after."
While other SACP members have previously confirmed Mr Mandela's party membership, many of their testimonies were given under duress in police interviews, where they might have sought to implicate him. However, the minutes from the 1982 SACP meeting, said Prof Ellis, offered more reliable proof. "This is written in a closed party meeting so nobody is trying to impress or mislead the public," he said.
Although Mr Mandela appears to have joined the SACP more for their political connections than their ideas, his membership could have damaged his standing in the West had it been disclosed while he was still fighting to dismantle apartheid.
Africa was a Cold War proxy battleground until the end of the 1980s, and international support for his cause, which included the Free Nelson Mandela campaign in Britain, drew partly on his image as a compromise figure loyal neither to East nor West.
"Nelson Mandela's reputation is based both on his ability to overcome personal animosities and to be magnanimous to all South Africans, white and black, and that is what impressed the world," said Prof Ellis, a former Amnesty International researcher who is based at the Free University of Amsterdam. "But what this shows is that like any politician, he was prepared to make opportunistic alliances.
Nelson Mandela 'proven' to be a member of the Communist Party after decades of denial - Telegraph