Stephen Hawking and a group of other leading British scientists have called for a posthumous pardon for Alan Turing, the Enigma codebreaker convicted of homosexuality in 1952 which was then a criminal offence. Two years later, he committed suicide.

Describing Turin as ``one of the most brilliant mathematicians of the modern era’’, they said that his work at Bletchley Park in breaking the German Enigma code was widely credited with shortening the Second World War.

"Yet successive governments seem incapable of forgiving his conviction for the then crime of being a homosexual, which led to his suicide, aged 41,’’ they said in a joint letter to The Daily Telegraph urging Prime Minister David Cameron to exercise his authority and formally forgive ``this British hero’’.

The 11 signatories , who include some of the biggest names in British science and politics, pointed out that Turing was a national ``hero’’ whose pioneering work remained relevant even today.

The move came after Lord Sharkey, the Lib Dem peer and one of the signatories to the letter, introduced a private members bill in the House of Lords to grant Turing an official pardon this summer.

In 2009, the then Labour Prime Minister Gordon Brown made a posthumous apology describing the way Turing was treated as "appalling". But he was not officially pardoned.

Earlier this year, an appeal for pardon was turned down by the Government. The Justice Minister Lord McNally admitted the case was "shocking" but said a pardon was "not considered appropriate as Alan Turing was properly convicted of what at the time was a criminal offence".

The letter, signed among others by Lord Rees of Ludlow, the Astronomer Royal; Sir Paul Nurse, the head of the Royal Society; and Baroness Trumpington, who worked for Turing at Bletchley Park during the war, said that 58 years after Turin’s death ``it is time his reputation was unblemished’’.

``To those who seek to block attempts to secure a pardon with the argument that this would set a precedent, we would answer that Turing’s achievements are sui generis,’’ the signatories said.

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