The men, who have been variously accused by the US authorities of attending terrorist camps, volunteering for suicide missions and pledging allegiance to Osama bin Laden, are thought unlikely to face prosecution here.

The Government has, however, given Washington firm assurances that it will monitor the activities of the men very closely. That is likely to mean almost constant surveillance by the intelligence services.

Whitehall sources confirmed extensive dossiers had been compiled on all the men by MI5 agents who interviewed them at Camp Delta. The dossiers would not be admissible evidence in British courts because the detainees were neither cautioned nor legally represented.

Feroz Abbasi, from south London, Moazzam Begg, from Birmingham, and Martin Mubanga and Richard Belmar, both from north London, will be arrested under the Terrorism Act 2000 when they are flown into London.

They will be taken for questioning to the high-security Paddington Green police station where they can be held without charge for up to seven days.

The interviews will be conducted sensitively because of fears that the men, who have alleged that they have been tortured and been held for periods in solitary confinement, are thought to be in poor mental health.

The men have claimed that admissions they are alleged to have made were extracted under duress. “The Americans may want to share intelligence with us,” said a police source. “But the anti-terrorist branch can only act on evidence.”

The deal to grant the men’s freedom, which Tony Blair had personally requested from President Bush, was not agreed until Britain gave the Americans firm promises on surveillance.

Jack Straw told the Commons yesterday that the release of the four followed “intensive and complex discussions to address US security concerns.” The issue of charges in Britain would be dealt with by the police and Crown Prosecution Service.

Mr Straw said: “I should like to assure the House that every practical step will be taken by the relevant UK authorities to maintain national security and to protect public safety.”

Downing Street reiterated that the issue had been crucial. “There are genuine security concerns which we have striven to meet,” it said.

Mr Blair and Mr Straw have lobbied the US Adminstration at the highest levels for the release of the detainees. Much of the detailed work in securing their freedom was done by Lord Goldsmith, the Attorney-General, in meetings with Pentagon lawyers.

The return of the four Britons would almost certainly still be in doubt without the US Supreme Court rulings last June which undercut the Bush administration's case for holding detainees incommunicado and indefinitely. It ruled that the US could not hold foreign nationals under such conditions and that Guantanamo Bay was, in effect, US soil, giving detainees a right to their day in court.

The surprise decision threw the centrepiece of the administration's war on terror policy into turmoil. Washington has still not decided how to react.

Lawyers and relatives of the four men said their primary concern was for their psychiatric health.

Louise Christian, solicitor for Abbasi and Mubanga, said: “The five who came back last year were severely traumatised and these men have suffered worse conditions for longer periods. I fear what state they are in.”

Four of the five men freed last year are suing the US authorities for $22 million. The prisoners to be released in the next few weeks may consider similar legal action.

The families of the four detainees were informed by the Foreign Office of the decision to release them before Mr Straw made his statement.

Agnes Belmar, mother of one of the men, said: “He is not home yet, so I am still a bit anxious.” Azmat Begg, a prominent campaigner for his son’s release, said he expected him to be interviewd by police. He said: “I am not against any procedure. I am a loyal British citizen.”

Sir Menzies Campbell, the Liberal Democrat Foreign Affairs spokesman, told MPs the men were about to be rescued from a “legal no man’s land”.

He said their detention violated all legal principles, their civil rights were systematically abused and they were denied due process, a cornerstone of the American legal system.

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