Johannesburg - A South African painting worth millions has been discovered in a London flat where it was being used as a kitchen notice board, fine art and antique auctioneer Bonhams said on Wednesday.The 76-year-old painting by artist Irma Stern titled Arab in Black was discovered earlier this year by Bonhams's head of South African art, Hannah O'Leary."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," O'Leary said in a statement.The painting was put up for auction in the early 1960s to raise money for the lawyers representing former president Nelson Mandela and his co-accused during the Treason Trial which began in 1956 and ended in 1961.A Treason Trial Defence Fund had been set up to raise money for their legal fees and to support the defendants' families, Bonhams said.Stern had donated a work to the cause.The painting was originally owned by Betty Suzman, an art collector and daughter of Max Sonnenberg, who founded Woolworths South Africa in 1931.Suzman was related to anti-apartheid activist and former MP Helen Suzman through marriage.The artwork found its way onto British shores when its buyer emigrated to the United Kingdom in the 1970s, Bonhams said. It was subsequently bequeathed to its current owner.The painting's estimated value is between R13m to R20m. It will go on sale on September 9 in London.