Gordon Brown secured a slender Commons victory tonight when MPs voted to approve highly controversial plans allowing terrorist suspects to be detained without charge for up to 42 days.
After a day of suspense and intrigue, the government won by a majority of nine. But, in a blow to Brown's authority, he won only because the Democratic Unionist party's nine MPs decided at the last minute to back Labour.
On the crucial division, the government won by 315 votes to 306, with 36 Labour MPs voting with the opposition. That made it Brown's biggest revolt since he became prime minister just under a year ago.
The DUP denied trading their votes in return for concessions from the government, although the Tories were claiming in Westminster that government spending in Northern Ireland would rise by £1.2bn as a result of some unofficial deal for support.
Downing Street denied that any deal of that kind had been done.
The result will be a relief to Brown. Following Labour's disastrous performance in the local and London elections and in the Crewe byelection, and with Brown's personal performance ratings at a historic low, there had been speculation that a defeat could be the trigger for a challenge to his leadership.
But some MPs believe that his standing has still been damaged by the vote, which saw wavering backbenchers coming under unprecedented pressure to support him.
Tory MP Ann Widdecombe and Ukip MP Robert Spink also voted with the government.
Ministers now face a considerable battle to get the proposal through the House of Lords, where the Tories and the Liberal Democrats have a majority over Labour and where senior legal figures, including the former lord chancellors Lord Falconer and Lord Irvine and the former attorney general Lord Goldsmith, are expected to lead the opposition to the bill.
The counter-terrorism bill would give the police the right to detain terrorist suspects without charge for up to 42 days on one-off occasions where there was a severe terrorist threat. The current limit is 28 days, which is itself a quadrupling of the seven-day limit that existed in 1997.
To persuade the backbenchers to support the bill, ministers offered a series of concessions which would involve the Commons having to vote to approve a decision to let police go beyond 28 days. Jacqui Smith, the home secretary, offered a further olive branch today when she said that suspects held beyond 28 days and subsequently released could qualify for an unspecified amount of compensation.
With less than two hours to go before the vote Downing Street admitted that its latest advice from the Labour whips' office was "if the vote was to take place now, the government would not have enough votes to win".
The belief that the vote would be extremely tight was reinforced by the DUP's reluctance to say whether its members were going to support the government.
One Labour MP backing the PM suggested some Tory MPs expected to vote with the government were being encouraged by their whips to go home before the vote took place.
The prime minister's spokesman sought to play down suggestions that Labour rebels were being persuaded to support Brown with promises of a policy changes, such as dropping sanctions against Cuba and offers of £3,000-a-day compensation for suspects unlawfully detained under the new law.
He would only say Britain's position on Cuba was an issue "kept under review" and that since the sanctions were imposed by the EU it was up to the EU presidency to decide whether to take a fresh look at them.
On compensation for terror suspects, the spokesman said that people who had been imprisoned unlawfully could already seek redress through the civil courts.
"It's not a great issue of principle at stake here," he said. However, he did confirm the Home Office was in the process of working out details of how suspects detained under the proposed laws could be compensated.
Mohammad Sarwar, the Labour MP for Glasgow Govan, decided to back the government after he was told that anyone locked up for as long as 42 days and then released without charge would receive compensation on a day-by-day basis.
After the vote Nick Clegg, the Liberal Democrat leader, said that the result was "a sad day" for British liberties. "What we have seen is the sacrifice of principle in order to save the skin of a sinking prime minister. It has nothing to do with the substance of the matter," he told Sky News tonight.
David Davis, the shadow home secretary, said: "We won the argument, the government bought the votes. Ministers utterly failed to provide the evidence in favour of 42 days in the Commons, and the measure is likely to be rejected in the House of Lords.
"Amidst widespread reports of vote-buying, the government did not have the Labour support to win - leaving its parliamentary authority in tatters."
Shami Chakrabarti, the director of civil liberties group Liberty, said: "Whilst disappointed, Liberty pays tribute to the brave parliamentarians of all stripes who held their nerve against the pressures of party politics and the terrorists' attempts to provoke us to abandon our values.
"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."
John McDonnell MP, the chairman of the left-leaning Labour Representation Committee, said: "Any attempt to present this as some sort of victory for the government will ring absolutely hollow.
"There will be widespread consternation among our supporters in the country seeing a Labour government prepared to use every tactic available in its determination to crush essential civil liberties, which have been won by the labour movement over generations.
"This is no way to run a government. Securing votes by threats, bribes and personal pleading demeans the role of the prime minister. Backbench Labour MPs from all sides of the party have looked on in disbelief at how the government has mishandled this issue.
"This is not the end of the battle over 42 days, merely the beginning."
Amnesty International's UK director, Kate Allen, said: "This is a dangerous and disappointing decision. No government minister should have the power to allow police to lock people up for six weeks without charge.
"Amnesty will now take the fight to the Lords. These are important human rights that are being taken away and we will do all we can to protect them.
"People have a right to be charged promptly or to be released; it's shocking that the law in the UK is moving further and further away from this basic principle."
Following the crucial vote, the government won a second vote during the report stage of the bill by a majority of 21. The counter-terrorism bill was then given a third reading by 315 votes to 78.
At prime minister's questions earlier today, Brown urged MPs to back 42-day detention without charge for terrorism suspects or face the risk of having to rush through emergency measures "in a moment of panic".
Squaring up to his opponents, Brown told critics he would take "no risks with security".
"Every senior policeman and every senior member of the security services have convinced me that an extension to 42 days' pre-charge detention is needed," Brown said. "I don't want, in a moment of panic, to come to the house for emergency measures."
But the Conservative leader, David Cameron, insisted prosecutors and security services believed the new powers were unnecessary.
In a fierce Commons clash, he branded the government's plans "unworkable" and a "symbolic assault on liberty that is unnecessary".
"Isn't it clear the terrorists want to destroy our freedom and when we trash our liberties we are doing their work for them?" Cameron asked.
Brown hit back, insisting: "It's no good to have opposition for opposition's sake. We have to take no risks with security."