Gordon Brown made a last-ditch appeal to Labour backbenchers today to help him to push through terrorism proposals upon which he has staked his reputation, as Downing Street warned that it did not yet have the numbers to win tonight’s vote.

Making a direct plea at Prime Minister's Questions, Mr Brown said that MPs would be failing in their duty to protect national security if they did not support plans to extend detention-without-charge for terror suspects from 28 to 42 days.

Later, opening the debate, the Home Secretary Jacqui Smith said that the extension was necessary because terrorists were more "ruthless" than ever before and police needed longer to investigate their plots.

However, attacking the plans, David Cameron accused Mr Brown of "trashing" British civil liberties, claiming that there was insufficient evidence that they were needed. He added that the Prime Minister had been guilty of making so many compromises and changes to the legislation that it had become impossible to implement.

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"Isn’t it clear that terrorists want to destroy our freedom, and when we trash our liberties, we do their work for them?" Mr Cameron demanded.

Appealing to Labour MPs to vote with their consciences and reject the legislation, he added: "This is not about the future of our Prime Minister. This is about our liberties."

Mr Brown responded with a withering attack on the Tories, and Labour rebels, for failing to face up to the terrorist threat.

"Our first duty is the protection of national security. We fail in our duty if we do not take preventative measures," he said. "I say in sorrow rather than anger, it is no use opposition for opposition’s sake. We have to take no risks with security."

As the tension increased both parties used different strategies to bring out their MPs. The Tories deliberately played down expectations by conceding that the Government was likely to win, which was seen as a ploy to get as many MPs to attend the 6pm division as possible.

The Prime Minister, meanwhile, tried the opposite tactic, with his spokesman telling the regular afternoon briefing of political journalists that he would lose if the vote took place now — a move designed to make sure wavering MPs support the Government to prevent a Tory victory.

"The advice from whips to No 10 remains that if the vote were to take place now, the Government would not have enough votes to win," the spokesman said.

Meanwhile, divisions on the Conservative position were beginning to appear for the first time today, with the website representing grassroots Tories, conservativehome.com, criticising the party for rejecting Mr Brown's proposals.

"A clear majority of the British people favour a longer detention period. We believe that the British people are right," it said, in an editorial.

"They won't readily forgive any politicians who allow a major atrocity to occur because our detention procedures prove to be inadequate."

The latest compromise deal, unveiled by Tony McNulty, the Home Office Minister, would have terrorist suspects who are held for longer than 28 days but then released given compensation for every day they were held after the 28th day. The Home Office refused to confirm a specific figure but reports suggested that it could be £3,000 for every day a suspect spent in custody.

The Prime Minister is thought to have sanctioned the compromise after being shaken by his narrow victory, by just 23 votes, on a far less controversial measure last night. The Government won by only 310 votes to 287 after the failure of a cross-party bid to thwart "secretive" inquests being held without a jury in sensitive terror cases.

In a sign of how tight the situation is — with The Times reporting today that Mr Brown was relying on opposition support to save himself — David Miliband, the Foreign Secretary, has cut short a visit to Israel to attend the crucial divisions in Westminster, while whips were believed to be encouraging sick MPs to come to the Commons and attend the vote.

A revolt by about 34 Labour MPs would be enough to defeat the Government if all the opposition parties voted against it, meaning that the result may hinge on the Ulster Democratic Unionists who are expected to meet today decide which way their nine MPs will vote.

The former Cabinet minister Frank Dobson, who says that he plans to rebel, said: "I am not going to question anybody’s motives. All I am saying is that I believe this is a quite fundamental issue: we have had the right not to be locked up for a long time without charge since the year 1215 — this is one of the very few things that is actually in Magna Carta and we need to sustain it."

The debate is ongoing, and will conclude with a vote at 6pm.

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