Gordon Brown has ruled out resigning if he loses the 42-day anti-terrorism vote as ministers and whips unleashed a fierce campaign to dissuade Labour backbenchers from opposing the Government.

The day after the Prime Minister put his authority on the line by refusing any concessions over the 42-day detention plan, he insisted that the vote next week would not be an issue of confidence.

He said that the proposal would be put to Parliament “in the normal way”, meaning that it would not be a confidence vote. Ministers recalled that Tony Blair also had not made defeat on the 90-day detention proposal a resignation matter. For Mr Brown to have done so would have been perilous, given that the outcome looked to be on a knife-edge last night.

Despite Mr Brown’s defiance, ministers and whips are agreed that the Prime Minister is so weakened after recent setbacks that a Commons defeat would further erode his battered authority and throw the Government into turmoil. Referring to yesterday’s article by Mr Brown in The Times, in which he refused to water down the 42-day idea, a whip said: “He has decided to stand by his principles. That is all very well but we now have to save him from himself.”

Ann Widdecombe, the Tory former Home Office minister, said that she was prepared to vote with the Government as long as there was a “sunset clause”, requiring the legislation to be renewed each year by Parliament.

Jacqui Smith, the Home Secretary, told Labour MPs last night that there would be concessions to tighten safeguards around the 42-day plan, which will be published today. As the party’s MPs returned in a grim mood from the spring recess, however, whips warned them that, if they insisted on defeating the Prime Minister, the impact on Labour’s already slim long-term chances could be catastrophic. “We are being told that disunited parties will lose and that we should swallow our pride and come on board,” one firm rebel said.

Speaking to a packed meeting of the Parliamentary Labour Party, which was not attended by Mr Brown, the Home Secretary promised three sets of concessions: reducing from about 30 days to a week the period during which Parliament has to approve detention powers; cutting from 60 days to 30 the period during which emergency powers could be used; and defining more precisely the conditions that would amount to an emergency.

It is understood that the “exceptional circumstances” needed to trigger the 42-day powers would be closely modelled on existing emergency legislation – a key Conservative demand.

Ms Smith said after the meeting that she had detected clear movement among MPs towards accepting the proposals. She said that she had told them that the country expected them to take the necessary steps to defend national security.

Two MPs, Jim Devine and Richard Burden, are understood to have declared that they had heard enough to persuade them to vote for the Government next week. Others, including Keith Vaz, chairman of the Home Affairs Select Committee, and Andrew Dismore, chairman of the Joint Committee on Human Rights, will wait to see the detail of the concessions. Chris Mullin, a former Foreign Office minister, and Jeremy Corbyn and Paul Flynn, both of whom are serial rebels, spoke against the measures.

A potential lifeline for Mr Brown emerged last night from the Democratic Unionists. If their nine MPs abstain or vote for Mr Brown, his chances of defeat will be reduced.

David Davis, the Shadow Home Secretary, said that trying to increase precharge detention had turned into a “horrible political cul de sac” for the Prime Minister. In an interview with The Monitor he said: “I think the logic is probably something like this: Tony Blair was defeated on 90 days; and most of the people who voted against him were Brownites.

“Therefore, Brown thinks they will support him if he comes back with more of a compromise and tries to present it as a consensual argument: ‘I am likely to succeed where he failed’.”

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