Tony Blair suffered a devastating blow to his authority last night as Labour MPs helped to inflict the first defeat of his eight-year premiership by decisively voting down plans to detain terrorist suspects for up to 90 days.

Although Downing Street insisted that the 31-vote defeat was not an issue of confidence, Labour MPs said it raised fresh questions about how long Mr Blair could remain Prime Minister.

Michael Howard, the outgoing Conservative leader, said Mr Blair"s authority had been reduced to "vanishing point".

"This vote shows he is no longer able to carry his party with him," he said. "He must now consider his position."

Labour rebels had joined forces with Tory MPs and Liberal Democrats to vote against 90 days. Then they defied the Government for a second time to back a rebel Labour amendment for a maximum 28 days detention, double the present limit.

During angry exchanges at Prime Minister"s Questions, Mr Howard argued that the Government had failed to justify the need for 90-day detention and said it could alienate ethnic communities.

Within an hour of the defeat, a shaken but unrepentant Mr Blair summoned the cameras to No 10 to declare that he had no intention of quitting. He said that public opinion was on his side and that Parliament had "made the wrong decision - the country will think that Parliament has behaved in a deeply irresponsible way".

Asked if he would quit, he said: "Not on the back of this. It is better sometimes to lose doing the right thing than to win doing the wrong thing."

He had made the same comment at Prime Minister"s Questions after Labour whips warned him that he was heading for his first Commons defeat since coming to power in 1997.

Mr Blair said it was his duty to support the police, who had asked for a 90-day period to allow them to collect the evidence needed to bring effective cases against terrorists.

The defeat was greeted with raucous cheers by the resurgent Tory party as Mr Blair and fellow ministers sat grim-faced opposite.

Mr Blair"s official spokesman claimed that the defeat was a "one-off issue" and said the Government would respect the decision of the Commons to double the period terrorist suspects could be held from 14 to 28 days.

But the Labour Party was in ferment, with Mr Blair"s long-standing critics and loyal MPs fearing that the vote was a watershed for a Prime Minister who had been dubbed "Teflon Tony" for his vote-winning abilities.

A leading Blairite MP said the Government was now in "uncharted waters", as the defeat had raised questions about Mr Blair"s judgment, his grip on the party and how long he could remain at No 10 before handing over to Gordon Brown, the Chancellor.

Mr Blair"s backbench critics were harsher, calling for him to resign and hand over to Mr Brown.

Paul Flynn, a prominent opponent of 90-day detention, said the vote showed that Labour MPs were no longer "poodles". Clare Short, who quit the Cabinet after the Iraq war, said she hoped that the defeat would "speed Mr Blair on his way. It would be good for the country and the Labour Party if he moved off".

Mr Blair"s decision to press ahead with a vote on 90 days was seen as one of the most serious miscalculations of his premiership.

Charles Clarke, the Home Secretary, had hinted last week that the Government would seek a compromise on a lower limit, which Labour MPs had believed would be around 42 days. That could well have secured the support of many Tory MPs.

But Mr Blair overruled Mr Clarke and insisted that there could be no compromise. He authorised an unprecedented lobbying exercise, with the Home Office telling chief constables to contact their MPs to put the case for 90 days.

What will be seen as one of Mr Blair"s blackest days began with the news that the Chancellor and Jack Straw, the Foreign Secretary, had been summoned back from overseas visits for the vote. A furious Mr Brown was hauled back to London within minutes of arriving at Tel Aviv for a peace conference.

Intense arm-twisting by whips and personal appeals for loyalty from Cabinet ministers failed to budge the rebels, who protested that Mr Blair was pushing his party too far. Forty-nine Labour MPs, including 11 former ministers, voted against the Government in the biggest rebellion of this parliament.

Labour MPs said that Mr Blair faced further - and potentially more serious - rebellions over controversial proposals to increase the role of the private sector in health and education and plans to update the Trident nuclear deterrent.