The families of the other Britons are said to be stunned by James Lee’s admission and claim that it has ruined any chance that the men have of proving their innocence. One legal source said: “The Saudis take the view: ‘One guilty, all guilty.’ ”

British diplomats in Riyadh said that they were astonished by Mr Lee’s written confession and his plea for clemency at the weekend and are demanding urgent talks with the Saudi authorities over their next move.

The six other detainees, who are accused of killing two other Britons in bombing attacks, staged allegedly to protect their lucrative trade in alcohol, have not been told of their colleague’s confession. They insist that they want to fight to clear their name and their relatives are now asking to meet Jack Straw, the Foreign Secretary, after a secret meeting at the Foreign Office in the week before Christmas when, they say, they were promised a “new initiative” to win the Britons’ release.

Amnesty International is among the human rights groups that have condemned the British Government for its handling of the case. A senior British diplomat admitted in the meeting with the families that the Saudis were making a laughing stock of Whitehall over the issue.

Mary Martini, whose former husband, James Cottle, is among those being held, asked: “How many more of the men will the Saudis break before our Government does something to help them?” In recent weeks the other British detainees told diplomats that Mr Lee, 40, a hospital engineer from Cardiff, had suffered a breakdown while in prison. He was said to have kept bursting into tears and had threatened suicide.

Mr Lee was moved suddenly from the al-Hajr high security prison to the secret police headquarters in Riyadh without the British Embassy being told.After his arrest two years ago, Mr Lee was paraded on television with two other Britons and was heard confessing to planting the bomb on a car driven by David Brown in December 2000, which blinded and maimed the Coca-Cola executive.

Days after the televised confessions, Mr Lee and the other Britons retracted their statements, saying that they had been tortured, and they appealed against the convictions and their 18-year jail sentences. Mr Brown has since said that he does not believe that the jailed Britons were behind the attack and blames Saudi militants with links to al-Qaeda.

Two of the Britons were sentenced to death for their part in the murder of Christopher Rodway, an engineer who was killed in a car bomb attack in November. A second man, Simon Veness, was killed in another explosion June 2002.

The appeals were heard last October, but the Saudis refused to reveal the verdict.

One diplomatic source said: “Now that Mr Lee has confessed, the Saudis can say all the Britons are guilty and ask for blood money for their victims.” The family of Mr Rodway has said that it does not want any money.

Relatives of the Britons say that they had been assured that a senior minister would visit Saudi Arabia later this month to discuss their plight.

Baroness Symons of Vernham Dean, the Minister for International Trade, is said to be going to the kingdom on January 18 with a delegation of British executives and “may try” to contact Saudi officials about the detainees. The Government, however, does not want a diplomatic row with the Saudis in the approach to a possible war with Iraq.

Saudi ministers could now face months of wrangling over how much compensation the Britons should pay relatives of the dead and injured before they are freed.

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