London — Prime minister Brown narrowly scraped home last night in a crucial vote on 42-day detention for terrorism suspects.

Just nine votes saved the Mr. Brown from an embarrassing defeat as he survived a late backbench rebellion.

Mr. Brown had staked his authority on winning the vote, calling dozens of Labour Members of Parliament to persuade them to back his proposal to increase the time that terrorism suspects can be held without charge.

If the measure becomes law, Britain will have the toughest anti-terror laws in the Western world.

The prime minister’s victory depended on the support of the MPs from the Democratic Unionist Party. Mr. Brown — who will attempt to use the win to re-establish his personal credibility — was accused of doing backroom deals with the Ulster MPs to save his skin.

If the nine DUP MPs had voted against the legislation, it would have been defeated. If they had abstained it would have required the casting vote of the speaker to pass the legislation to raise the limit from 28 days.

The narrowness of Mr. Brown’s victory calls into question the Government’s mandate for introducing such a controversial law. However, recent opinion polls indicated the public was overwhelmingly in favour of the change.

A Daily Telegraph/YouGov poll showed that 69% accepted the need for the 42-day detention limit in “exceptional circumstances.”

Last night details emerged of what the DUP had secured for voting with the government. Householders in Northern Ireland are due to begin paying water rates at levels equivalent to those in the rest of Britain from 2010, rising next year to $500 from $320. The DUP has won an agreement that the $400 million this will raise annually will be spent on taxpayers in Northern Ireland.

In addition, the Northern Ireland Secretary, Shaun Woodward, reassured the DUP that the government had no plans to end the ban on abortion in the province. Abortion has never been legal in Northern Ireland.

Thirty-six Labour MPs voted against the change. The Government’s majority of 65 was reduced to nine, with 315 MPs in favour and 294 against. The former minister, Ann Widdecombe, was the only Tory MP to vote with the government.

A senior DUP MP, Willie McCrea, defended his party’s decision to back the government.

“We decided on an issue of principle on national security,” he said. “Northern Ireland knows more than any other part of the U.K. what the ravages of terrorism can do. We could not in any way put our hands to something that would endanger the U.K.”

The vote was considered the biggest test of Mr. Brown’s year-long premiership and he will hail the victory as one for improved British security.

The leader of the Commons the Conservative Party, David Cameron, said the proposals were “draconian” and “ineffective.”

Mr. Brown said the proposal was “not only popular” but “necessary and right.”

After the vote, the prime minister congratulated the Labour whips, and he was seen kissing the Home secretary, Jacqui Smith, who had been piloting the laws through the Commons.

He also thanked the Ulster Unionist MP, Lady Sylvia Hermon, who also voted with the government.

Opposition parties and civil liberties groups argued that the new rules were a step too far. The shadow Home secretary, David Davis, said: “We won the argument, they bought the votes. It wasn’t the argument that won the day it was the [Labour] whips that won.

“Gordon Brown can’t be proud today.”

The director of civil rights group Liberty, Shami Chakrabarti, said: “Recent years have shown how forgetting Britain’s moral compass has left our country less safe; so on to the House of Lords — once more the guardian of fundamental rights.”