Gordon Brown appeared last night to be relying on opposition support to save himself from defeat in today’s cliffhanger vote on detaining terrorist suspects for 42 days.

Labour whips warned the Prime Minister that not enough Labour MPs were on board to guarantee him victory, as calculations by The Times suggested that at least 40 of them were determined to vote against the Government.

If all MPs from the opposition parties turned up to oppose the Government, only 34 Labour MPs would be needed to defeat it. That has left Mr Brown and the whips hoping that at least some of the minority party MPs would either not be at Westminster today for the 6pm vote or that, even at the eleventh hour, the nine Democratic Unionist MPs could be persuaded to change their stance.

In 2005 they voted against the plan to hold suspects for up to 90 days, and Downing Street admitted last night that it was not “working on the assumption” that they would help out the Government. Even so, the party appeared to be in the market for a deal. It is known that they have been spoken to by ministers and Conservative spokesmen, but no agreement appeared to be on the table last night.

In an obvious sign of the Government’s difficulties, its majority was cut to 23 last night in the first challenge to its counter-terrorism Bill. A cross-party bid to block plans for “secretive inquests” to be held without a jury in sensitive terror cases was defeated by 310 votes to 287.

Mr Brown continued to talk to Labour MPs. Some senior backbenchers believe that he deserves credit for not running away from the vote when assessments by the whips told him clearly that he was in danger of losing. Others believe that a defeat would weaken further Mr Brown’s authority.

Labour MPs admit that they are being told by whips and ministers that voting against the Government would be handing ammunition to the Tories and putting the party on the wrong side of public opinion. Polls suggest that voters support the 42-day plan. Labour MPs are also being told that they can afford to pull back from opposing the suggested detention power because it will be blocked in the House of Lords.

A few Tory MPs are known to have reservations about opposing the power but they are coming under pressure from their own whips not to throw away the chance of defeating the Government. The Conservative whips even appealed to one MP whom David Cameron recently ejected. Bob Spink, the MP for Castle Point who left the Tories after a row over his constituency, said that one of his former colleagues had asked him to help to “embarrass the Government”. Mr Spink indicated that he was considering voting for the measure, however.

The Labour whips have cancelled all leave for the vote. David Miliband, the Foreign Secretary, was brought back early from a trip to the Middle East and MPs who have been unwell have been urged to travel to Westminster.

If Mr Brown loses, he will try swiftly to turn attention to other issues. If he wins, he will be given an important breathing space in his fight to restore his reputation, but the battle over 42 days will go on in the Lords, where former Cabinet ministers are expected to carry on the opposition.

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