Tony Blair suffered his first ever Commons defeat today when almost 50 Labour rebels defied the whip to throw out a government proposal for terror suspects to be held for up to 90 days without charge.

MPs voted by 322 to 291 against a government amendment to the Terrorism Bill which would have raised the current maximum detention period from its current 14 days. A total of 49 Labour MPs voted against the Government, far more than ignored the whips in a mini-rebellion last week on the same issue.

The result of the vote was greeted by raucous cheers from the Tory benches while the Prime Minister shook his head in disbelief.

MPs were then asked to vote on an amendment tabled by the Labour backbencher David Winnick, which proposed a maximum detention period of four weeks, the timespan backed by the Conservatives. They supported that proposal by 323 votes to 290, forcing the Government to accept the amendment into its legislation.

It was Mr Blair's most stinging political defeat and will cast more doubt on both his political authority and his judgement after a bruising two weeks.

After trying, and failing, to outmanoeuvre his political opponents, the Prime Minister will also stand accused of needlessly politicising the debate on national security despite the attempts of the Opposition - and his own Home Secretary - to find a cross-party consensus.

Michael Howard, the Conservative leader, said: ""Mr Blair’s authority has been diminished almost to vanishing point. This vote shows he is no longer able to carry his own party with him - he must now consider his position."

And Alex Salmond, the Scottish Nationalist Party leader, referring to Mr Blair's high-risk tactics leading up to the vote, telling reporters: "The Prime Minister has just fallen off the high wire."

In heated Commons exchanges during the day, Mr Blair and Charles Clarke, the Home Secretary, had argued that the extended detentions, backed by the police and security forces, were essential to defeat the terrorist threat. The Tories and Liberal Democrats said that the proposal went too far and would be counter-productive and had instead backed a 28-day maximum.

As the Whips went into overdrive to try to salvage the detention plan, they took the rare step of recalling both Gordon Brown, the Chancellor, and Jack Straw, the Foreign Secretary, from high-profile trips abroad.

Embarrassingly for Mr Brown, though, five MPs considered to be "Brownites' joined the rebellion, the most prominent Geoffrey Robinson, the former Paymaster General, who abstained. Those voting against the amendment included a total of 11 former ministers.

Mr Blair said he regretted the result, but had no intention of resigning. "It is better sometimes to lose doing the right thing than be doing the wrong thing," he said.

He added: "What I cannot understand is how we can say, given the strength of the terrorist threat that we face, that the civil liberties of a small number of terrorist suspects - who we are saying in any event have to come back before a court every seven days - come before the fundamental civil liberty in this country of protection from terrorism.

"I think it was the wrong decision and I just hope in a later time we don’t rue it."

The defeat is the first for any Government since John Major's administration was defeated in an obscure fisheries debate in 1995. But the size of the rebellion suggests that it will not be Mr Blair's last - backbenchers have serious reservations about many items on Mr Blair's legislative agenda, including the introduction of ID cards and the corporatisation of education and health services.

The result will, however, give Labour the chance to project the Conservatives as being "soft on terror" - an accusation that visibly incensed Mr Howard, the Conservative leader, during Prime Minister's Questions at lunchtime.

An electrifying series of exchanges began with Mr Howard saying: "Every member of this House is united in a desire to take effective action against the new terrorist threat that we face."

Amid loud Labour jeers, he paused before adding, angrily: "There are people on the benches behind me who fought terrorism on the streets of Northern Ireland and that response is disgraceful."

The Opposition leader went on: "Since the Terrorism Bill was first presented to this House, it has been amended to provide for High Court supervision of detention, annual renewal and revisions to the code governing the questioning of suspects.

"Do you agree those changes, following as they do from debates in this House, represent an improvement to the Bill?"

Mr Blair replied: "We have made changes in order to try to reach a compromise, it is true. But I want to explain to you and your Tory colleagues why I believe this is so necessary. This proposal did not originate with government. It originated with the police and those responsible for anti-terrorist operations in our country."

Mr Blair also told the House that the security forces had foiled two separate terror attacks since four suicide bombers killed 52 London commuters on July 7. He gave no further details.

In an attempt to save the day, leading Government figures including Peter Hain, the Northern Ireland Secretary, were seen huddling with members of Ian Paisley's Democratic Unionist Party, which has nine members in Westminster.

But Dr Paisley said after the vote that his group had opposed the amendment. "Blair’s authority has gone. He should have a 60-odd majority, he has lost that. He has not done well," he said.

"In no way could you put any sugar on that cake. Indeed it isn’t even a cake: it is a crust."