Irma Stern's Arab in Black is set to headline a sale of South African art at Bonhams London on September 9.
The lot has a fascinating history. It was sold at a charity auction during the 1960s to raise funds for Nelson Mandela's defence after he was accused of high treason.
Mandela was arrested in 1958 for his role in collecting signatures from those opposed to apartheid.
He was found not guilty at the trial's end in 1961, but was given a life sentence in 1964.
The painting was recently discovered during the valuation of a house in London, where it had been used as a notice board for many years.
Hannah O'Leary, Bonhams' head of South African art explains: "I was undertaking a routine valuation when I spotted this masterpiece hanging in the kitchen covered in letters, postcards and bills.
"It was a hugely exciting find even before I learned of its political significance."
Irma Stern is South Africa's most valuable artist, with her Arab Priest selling for a record £3m ($4.6m) at Bonhams in 2011.
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