Gordon Brown faces the first major defeat of his premiership as ministers and security chiefs admitted last night that they were losing the battle to detain terror suspects without charge for 42 days. The Government has failed so far to secure the necessary support for the measure, despite the Prime Minister throwing his personal authority behind it.

Ministers have been forced to draw up a list of major last-minute concessions, The Times has learnt, amid efforts to win more support for the measure.

Rejection of the move would also embarrass Scotland Yard, which pressed MPs for extended detention powers and was heavily criticised for lobbying politicians over previous proposals for detention of up to 90-days.

One person involved in the lobbying said it was clear that the message was not getting through, despite a series of one-to-one meetings being held by Jacqui Smith, the Home Secretary, with backbench Labour rebels.

A senior Whitehall source admitted that the effort to establish the case for 42 days was being damaged, ironically, by a string of terror convictions in the courts. “We are victims of our own success,” he told The Times. In the latest conviction, after a lengthy trial and legal proceedings, one of Britain’s most potent terrorist brainwashers was found guilty of soliciting murder and running terror training camps.

Mohammed Hamid, 50, who recruited and radicalised the four would-be suicide bombers who attacked London on July 21, 2005, will be sentenced next month.

The conviction of Hamid and six other men brought to 65 the number of terrorist suspects found guilty in the courts since the beginning of last year.

Since the power to detain suspects was extended to 28 days in July 2006, only 11 people have been detained for longer than the previous 14-day limit.

Mr Brown remains committed to extending the limit to 42 days and has included the power in new counterterrorism legislation that will be debated by MPs late next month.

Senior figures concede that the measure has little chance of clearing the Commons in crucial votes in May in its current form, however.

The concessions being prepared include strict curbs on the type of emergency under which police could hold suspects beyond the current 28-day limit and to make the power dependent on a vote by MPs within a week of an emergency. One Labour MP has already admitted working with the Government on an amendment that brings the time limit for parliamentary approval from 30 days to ten. That will fall to seven as ministers seek to cut the number of rebels.

A defeat would be an embarrassing blow to the authority of the Government — echoing Tony Blair’s Commons defeat over the attempt to extend detention powers to 90 days.

Police and security chiefs are deeply anxious that the loss of the legislation would undermine their battle against al-Qaeda-inspired terrorism at a time of growing success.They want the 42-day power in place to deal with any future case where they would have to arrest a large number of suspects involved in multiple, complex plots.

Sir Ian Blair, the Metropolitan Police Commissioner, said last year: “If you can see the epidemic moving towards you then you start to take precautions before it arrives.”

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