Nelson Mandela has launched a last-ditch attempt to prevent a London gallery using his name on an exhibition of prison sketches that he claims are forgeries.

The former South African president said he "strongly disassociates himself" from the show, Nelson Mandela at 91, due to open tomorrow at the Belgravia Gallery. Lawyers for Mandela, who celebrates his birthday on Saturday, said they had written to the gallery asking it to "desist immediately" but received no reply.

Bally Chuene, Mandela's legal representative, said: "He did not sign those artworks."

The Belgravia Gallery, which also has rights to Prince Charles's art, was unavailable for comment yesterday, but has previously said it carried out exhaustive research to prove its collection was genuine.

The dispute centres on a limited series of lithographs produced by Mandela, in collaboration with an artist, depicting Robben Island and the cell where he spent many of his 27 years in prison during apartheid.

The original signed works were bought by international celebrities in 2002 and 2003, with proceeds going to his charities for homeless children and Aids victims. But his lawyers contended last year that unauthorised reproductions with false signatures were being sold...