An unpublished letter written by Nelson Mandela from his prison cell on Robben Island is to be offered at Bonhams on September 14 estimated at £50,000-100,000.

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The letter was penned to Barbara Lamb, the daughter of his friend and fellow anti-apartheid activist, Michael Harmel. In it, Mandela sends condolences on the death of her father, news of which had only recently reached him.

At the time – October 1974 – the future South African president was 10 years into a life sentence, following his conviction for sabotage at the Rivonia Trial in 1964.

In the letter, Mandela reflects on his friendship with Harmel, freedom and his deep musings on faith, writing: “It has been said that faith is like an oak tree, it grows steadily but, once established, it endures for centuries.”

Estimate £50,000-100,000.