Click here for an extract from the leaked document

Gordon Brown is preparing for a climb-down on his plans to detain terror suspects for 42 days despite insisting that he would not compromise on national security, according to a leaked Whitehall document.

The document shows that Jacqui Smith, the home secretary, has prepared concessions including giving judges powers to impose “alternatives to detention” on terror suspects after the existing 28-day limit.

The move would permit suspects held in a heavily guarded police cell, such as Paddington Green police station, to walk free – while remaining technically subject to the 42-day process. They would spend the final 14 days tagged or under house arrest.

The disclosure of the confidential plan comes as Brown faces his first big test of public support since becoming prime minister, with this week’s elections for London mayor and local councils.

With the polls suggesting Labour’s Ken Livingstone could be defeated by Boris Johnson, the Conservative mayoral candidate, Brown has been warned that he faces a “Blairite plot” to destroy his credibility.

Brown will now be exposed to ridicule because he said last week that he would not be “pushed about” by Labour MPs.

While caving in to the revolt on the 10p tax rate, Brown insisted that he would stand firm on extending detention before charge. “It’s right on security issues – because you never compromise security issues – that we should go ahead with 42 days,” he said.

Elsewhere he said: “The issue is doing the right thing and doing the right thing even if you were not to win a vote would be more important.” It is not yet clear whether the concession will be enough to buy off the rebels. However, critics are likely to assert that the flagship bill has been neutered to the point of meaninglessness.

Under the heading “Possible Concessions (not for disclosure)”, the leaked 90-page note says the proposal “could enable [a] judge to order alternatives to detention (tagging, bail etc)”, even though suspects remain technically subject to the 42-day process.

David Davis, the shadow home secretary, said the tagging concession did not go far enough and the Tories would continue to oppose the bill.

“This is a stepping stone to complete collapse, despite the prime minister’s protestations that he will not make any concessions,” Davis said. He compared the move to Brown’s backtracking on the 10p tax band.

Government supporters will argue the proposed tagging concession technically enables Brown to cling to his assertion that he is retaining a 42-day detention period. He will also be able to appease rebels who say it is wrong on civil liberties grounds for suspects to be kept that long in custody without being charged but backbenchers opposed to 42-day detention are likely to be emboldened to pressurise Brown to drop the plans altogether.

Brown’s authority was further questioned last night by Lord Levy, Tony Blair’s chief fundraiser. “I’m sorry to see there doesn’t appear to be that strong leadership the party needs,” he said.

Last week Kevan Jones, a member of the influential Labour parliamentary committee, told Brown at a private meeting the rebellion over the abolition of the 10p tax band was fuelled by diehard allies of Blair.

Ever since the 42-day plans were announced last year, ministers have insisted it was vital to have the powers to keep suspects behind bars. They said the plans were reserved for only the most serious terrorist suspects, for example those who might be involved in a hypothetical plan to copy the 9/11 attacks in Britain.

The new plan to give judges the power to tag or bail suspects after 28 days will demolish Brown’s insistence that it is vital for national security to detain suspects for longer than 28 days.

Tony McNulty, the counterterrorism minister, has hinted at some concessions. However, he has not disclosed that such powers would include the right to order the release of suspects on bail or tags — a proposal that effectively nullifies the point of extended detention.

The leaked note, marked “restricted”, was prepared for Smith last month by officials in the security and counter-terrorism office at the Home Office. Entitled “CT Bill Top Lines Briefing”, it provides Smith with a series of answers to questions she might be asked by MPs regarding the new bill. The next Commons vote on it is due in June.

A page outlining “possible concessions (not for disclosure)” explains the secret contingency plans drawn up to win over disaffected opponents.

In a section called “safeguards” the leaked note says the bill “could extend judge role in detention — could have statutory role in relation to welfare of suspect, could have same judge allocated throughout detention period, could enable judge to order alternatives to detention (tagging, bail etc). Would be resisted by CPS and police”.

Other concessions suggested in the note include an “independent commissioner” based at Paddington Green high-security police station. The opposition would be kept informed by the attorney-general.

Additional research: Hannah Gilkes

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