In the week in which Nelson Mandela visits Britain, sketches attributed to him are going back on sale after a gallery owner spent 19 months on a quest to prove that the artwork was genuine.

As the antiapartheid hero flies into Britain for his 90th birthday celebrations, finishing touches are being made to a London exhibition showing pictures of his island prison.

The works were withdrawn from sale after Mr Mandela sued in South Africa over concerns that proceeds had been diverted from charities and his signature may have been forged.

The allegations forced Anna Hunter, of the eminent Belgravia Gallery, who had bought 1,500 of the images, to turn detective to establish that her pictures were genuine.

“Accusations were made that Mr Mandela did not create these, which was awful - that there were fakes and forgeries around,” Ms Hunter said. “There was negative press in South Africa. We had purchased and paid for the works in full. At this point we decided to take them off the market to give us time to investigate the allegations.”

Ms Hunter had already sold hundreds of Mandela’s lithographs to buyers such as Frank Warren, the boxing promoter.

None demanded their money back. Instead, the gallery founder embarked on a jet-set exercise in “due diligence” which would lead her to Mr Mandela’s art teacher, his academic printer and a forensic handwriting expert.

Ms Hunter had been chosen to represent Mr Mandela’s artwork because of her work with renowned individuals such as the Prince of Wales.

The sketches were the idea of a Mandela adviser who believed that they could help raise money for the fight against Aids. The former freedom fighter returned to Robben Island, where he had spent 18 of his 27 years of imprisonment, with a photographer, Grant Warren. Mr Mandela then worked as a student with the artist Varenka Paschke, a granddaughter of the apartheid Prime Minister P. W. Botha, to produce lithographs.

“They tried various media to see how comfortable he would be with them, including charcoal which was difficult, watercolour, oil and then finally crayon,” Ms Hunter said. The former prisoner drew sketches based on the photos of Robben Island. These were overlaid with coloured acetates so that he could fill in the outlines. “She said it struck her that he was continuously drawn to using the brightest colours,” said Ms Hunter. Steven Inggs, Professor of Print-making at the University of Cape Town, turned them into limited edition prints. “He told me when they were producing these in the print room, other artists would say ‘Whose work is this?’. He’d reply ‘It’s work by Nelson Mandela’ and they’d say ‘Pull the other one’.” The pictures sold originally for £2,000 but now have price tags up to £15,000. Ms Hunter said: “Mandela’s work is not only technically brilliant - akin to lithographs by the likes of Braque and Picasso - but also historically of unprecedented importance.”

Mandela on . . .

The Queen “In public she is very stiff but when I stayed at Buckingham Palace she was a totally different person. She served tea herself and she become really mothering”

Margaret Thatcher “When I saw Margaret Thatcher for the first time we were supposed to have a meeting for one hour. Our meeting lasted for three hours and I had to offer an excuse. I then went to Neil Kinnock at Parliament and he was very excited. He said, ‘How is the Iron Lady?’. I said she was warm and motherly. He said, ‘Warm and motherly? You must have met another lady’ ”

The Prince of Wales “He’s a very fine chap. I went with him when I became state President to some area where the majority of people are blacks [Brixton]. He was more popular than I am. People passed me and shook hands with him because he is a very humble chap. We had to be taken away because the crowd became riotous. Everyone wanted to greet him and we were in danger of a stampede so they called the mounted police”

Bill Gates

“When he saw that I had formed a foundation he then invited us to stay with him. He gave [Mrs Mandela] $5 million because she also has a foundation and gave me $10 million”

Sources: Nelson Mandela video, Belgravia Gallery, London

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