For decades, it was one of the enduring disputes of South Africa’s anti-apartheid struggle. Was Nelson Mandela, the leader of the African National Congress, really a secret Communist, as the white-only government of the time alleged? Or, as he claimed during the infamous 1963 trial that saw him jailed for life, was it simply a smear to discredit him in a world riven by Cold War tensions?

Now, nearly half a century after the court case that made him the world’s best-known prisoner of conscience, a new book claims that whatever the wider injustice perpetrated, the apartheid-era prosecutors were indeed right on one question: Mr Mandela was a Communist party member after all.

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.

His book also provides fresh detail on how the ANC’s military wing had bomb-making lessons from the IRA, and intelligence training from the East German Stasi, which it used to carry out brutal interrogations of suspected “spies” at secret prison camps.

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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.”

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