The House of Commons' vote to quash Tony Blair's anti-terror legislation dealt the prime minister his first-ever defeat - an embarrassing setback that raises doubts about his political future. Blair had staked his authority on the plan to detain terror suspects for 90 days without charge and doggedly refused to compromise. But some 49 of his own lawmakers, including 11 former ministers, joined forces with opposition parties to defeat the measure Wednesday. Blair, whose popularity was dented by the divisive war in Iraq, was already under pressure from some in his governing Labour Party to step down. Now that rebels have inflicted their first defeat, some question how long he will survive as prime minister. ``The vote has substantially weakened his position,'' said Bill Jones, a political analyst at Manchester University, who noted Blair had upset many of his own lawmakers over the years by failing to listen to their views. ``You do get the sense that if he does not adjust his style of governing, he will be forced out.'' Michael Howard, leader of the main Conservative Party, said Blair's authority had ``diminished almost to vanishing point'' and said he should consider resigning. ``This vote shows he is no longer able to carry his own party with him. He must now consider his position,'' said Howard. But Blair was defiant and said lawmakers were wrong to extend the detention period to just 28 days. The current maximum detention without charge is two weeks. ``The country will think that Parliament has behaved in a deeply irresponsible way today,'' he said Wednesday. Blair was scheduled to meet his Cabinet Thursday morning and police and security services later in the day to discuss how to fight terrorism without the measure. Home Secretary Charles Clarke, the country's top law enforcement official, said the defeat did not have larger repercussions for the prime minister's authority. ``Yesterday was a combination of serial rebels, the opposition voting without respect for the issues, and a number of individuals profoundly worried about the civil liberties issue, voting that way,'' he told GMTV. Lawmakers voted 322-291 against the 90-day plan, and backed the 28-day period 323-290. The result is a humiliating blow to Blair. For eight years, his Labour government commanded an unassailable lead in the Commons and easily swatted aside opposition to its legislation - and crushed dissent within its own ranks. But Labour was punished in national elections earlier this year, largely over the Iraq war, and its massive majority of 161 seats more than all the other parties combined was cut to just 66. That has made the government more vulnerable to defeat, and put Labour rebels in a more powerful position. Labour left-wingers have long been unhappy with Blair's free market agenda of greater private sector involvement in health care and education. They may be emboldened by Wednesday's successful revolt and try to bloc a raft of legislation over the next 12 months. Rebel Labour lawmaker Paul Flynn said the party would no longer be Blair's ``poodle'' and called on the prime minister to set out a timetable for stepping down. ``Otherwise, we are going into a morass of division and conflict within the Labour Party,'' he warned. The prime minister has said he will not seek a fourth term in office. He could serve until 2010, but speculation is rife that he will quit sooner.