The letter was written to the daughter of his friend and fellow anti-apartheid activist Michael Harmel, and is estimated to be worth between £50 000 and £100 000 (R986 000).
The letter is addressed to Barbara Lamb and conveys his condolences on the death of her father. Harmel, who was a member of the SACP, died on June 18, 1974.
Mandela first met Harmel at a Communist Party meeting in the 1940s. In the letter he writes movingly about his initial failure as a young college graduate to appreciate the older man’s gifts: “I was convinced that he did not deserve the honour of being placed amongst the elite. It was some years later that I came to accept his simplicity as a virtue, on which one could model his own life”
In the letter, Mandela reflects: “He was one of those men who fully understood the meaning of their life as part of mankind generally and as individuals. His peep into the future very often coincided with one’s most intimate hopes and dreams. May he rest in peace, for his work on Earth is done.”
Mandela was sentenced to life imprisonment for treason following the Rivonia Trial in 1962. He served 27 years, split between Robben Island, Pollsmoor Prison and Victor Verster Prison.
The conditions under which Mandela lived when the letter was written were deplorable. Although by 1974 he had progressed from a Grade D to a Grade A prisoner, he was still sleeping on a stone floor, breaking stones in the yard every day, and was confined to his cell for 23 hours a day at the weekends.
Despite the conditions, Mandela maintained a sense of perspective and humour. Looking forward to his freedom, he promises to take his European “sisters” - close friends who had supported him in the days of the Struggle - to a feast and then to invite them to join in Umngqungpo, the Xhosa dance performed by elder women to celebrate girls who are coming of age.
Mandela ends the letter with a characteristically thoughtful interweaving of the personal and the philosophical. “It has been said that faith is like an oak tree, it grows steadily but, once established, it endures for centuries. Ever ridden a horse in your life, or seen a horse race? Hope is the horse on which you ride and travel to your destination, to reach the winning post. My only fortune in life is to have friends who taught me these things, amongst whom was your beloved Pa.”.
Bonhams director of the South African sale, Giles Peppiatt, said: “When Nelson Mandela wrote this letter, he had endured 10 years of appalling treatment with no prospect of release, yet he retained his humanity, his sense of humour and his faith in the future.
“He writes with almost conversational grace and ease. It is a wonderful letter.”
The auction of the letter is expected to take place in London.