Hostage negotiators feared that, after almost four months in captivity, Mr Kember, 74, might have been too frail to survive an SAS assault on the kidnappers’ hideout. The armed gang was given 15 minutes to flee, leaving the veteran British campaigner and two Canadian colleagues in safety.

A senior Western diplomatic source said: “The ultimatum was passed through an intermediary. It was a calculated risk but our priority was the safe return of Mr Kember and the two other hostages from the Christian Peacemakers’ Team (CPT). Our intelligence was that the kidnappers were not religious fanatics willing to die with their hostages, though we knew they were violent as they had by then murdered the American member of the CPT, Tom Fox. But we believed they were criminals whose main aim was to save their own skin.

“It is always a delicate matter how best to resolve such crises, and we get criticism, but we got the three hostages back, none of the rescuers was hurt, nor any innocent bystanders, and those responsible will be found,” the source said.

It had never been explained why the kidnappers abandoned their hostages minutes before the rescue team arrived.

The revelation that Mr Kember’s captors had been tipped off came amid a diplomatic row over allegations that the German Government had paid a substantial ransom to free two engineers this week.

German officials do not deny a suggestion by Alaa al-Hashimy, Iraq’s Ambassador in Berlin, that “a lot of money was paid”. Speculation abut a payment raised concern that, after a recent lull, criminal gangs might kidnap more foreigners for money. French and Italian authorities are said to have paid to free some of their citizens in Iraq.

British officials insist that no cash was paid to secure Mr Kember, from Pinner, northwest London. However, there is debate among Western diplomats whether it was right to let the kidnappers go. Iraq’s Interior Ministry is now responsible for finding them.

The fugitives belong to a notorious criminal gang with links to groups believed to have sold captives to militant Islamic groups. Intelligence officials say that the gang has collected millions of dollars in ransoms over three years and is thought to have done business with Abu Musab al-Zarqawi, al-Qaeda’s leader in Iraq. Al-Zarqawi is held responsible for the murder of the British engineer Ken Bigley and other Western captives.

Mr Kember was criticised by General Sir Mike Jackson, Chief of the General Staff, for not thanking publicly the troops who saved him. His wife, Pat, 70, received hate mail.

But soldiers involved in the rescue told how a jubilant Mr Kember posed for photographs with them. Pictures that show him hugging the SAS team now decorate a villa used by British Special Forces in Baghdad.

A military source close to the British-led rescue mission told The Times that rescuers were astonished at how quickly a suspect whom they had arrested on March 23 had betrayed the kidnappers’ safe house to MI6 interrogators.

The house was only a mile from the British Embassy but was surrounded by family homes, another factor behind the decision to try to resolve the crisis without a gunfight.

The rescue, at night, was led by 32 men from B Squadron of the SAS, backed up by 40 other British troops and heavy artillery. A series of trusted intermediaries used during weeks of negotiations ensured that word was passed to the kidnappers, who left the hostages chained together in a ground-floor room. The SAS team used stun grenades during the raid as they were not sure the kidnappers had gone.

British negotiators admit that they were losing hope of securing the release of Mr Kember, and the Canadian captives, James Loney and Harmeet Singh Sooden, after weeks of silence from their captors, who at no time sought a ransom.

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