Gordon Brown's chances of avoiding a humiliating defeat in the Commons this week dwindled after he effectively ruled out any more significant changes to proposals to detain terror suspects for 42 days without charge.

Senior ministers admitted last night that the Prime Minister was still short of the necessary votes to win the ballot on Wednesday despite a weekend of arm-twisting. “There's movement but we're not there yet,” one minister told The Times.

Although Mr Brown has made several concessions to the rebels in recent weeks, he rejected giving further ground in a letter that he sent to all Labour MPs on Saturday. Dismissing calls to make the measure subject to judicial review, he said: “I ... do not believe in this instance that this is the best way forward.”

Mr Brown may be hoping to save the concession for the House of Lords, however, and one minister did not rule out more changes “at the fringes” in the days to come.

About 50 Labour MPs have voiced concern about the 42-day proposal and between 35 and 40 would have to rebel to inflict defeat on Mr Brown, provided that he secures the support of the Democratic Unionist Party and some Conservatives.

Jacqui Smith, the Home Secretary, denied yesterday that a deal had been done with the DUP to secure the backing of their MPs in the Commons. Ms Smith said that the Government had “absolutely not” agreed to provide £200million of extra funding to the administration in Stormont in exchange for DUP votes.

The DUP, which has nine MPs at Westminster, has had behind the scenes meetings with both Labour and Conservative representatives. “My guess is that the DUP won't show their hand until 15 minutes before the vote,” said one of those involved in negotiations.

As all sides began turning up the pressure in the final days before the vote, David Davis, the Shadow Home Secretary, claimed that government whips had warned Labour MPs that they risked triggering a leadership crisis if they failed to support the Government on the issue.

Mr Davis said: “Whips have been ringing up the left-wing members of the Labour Party and saying, ‘Vote against 42 days on Wednesday and you get David Miliband on Thursday'. I have never heard a more ridiculous argument, but that is what is going on.”

Mr Brown's allies, preparing to protect the Prime Minister in the event that he suffers his first Commons defeat just a year after entering No10, will point to opinion polls that suggest the public supports his stance on detention.

Although a survey published in The Sunday Telegraph showed that support for Labour was at 26 percent, against 42percent for the Conservatives and 21 percent for the Liberal Democrats, the same poll by ICM found that 65percent of voters backed an extension to the detention limit.

Mr Brown's close Cabinet ally, Ed Balls, the Children's Secretary, told Sky News: “On the 42-day issue, we could very easily have ducked as a Government, walked away and said it is too difficult.

“It is about making the difficult choice and it is massively supported by the public in this poll.”

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