Tony Blair has made a career out of grand gambles and strategic surprises. Yesterday the great political thespian made himself up for another command performance. But there is a first time for everything. And after eight years in power, the Prime Minister now knows the sting of a Commons defeat. He tried to defy the scriptwriters, the political commentariat busily predicting his demise; Labour MPs sniping at his authority; and the commonsense advice of his leading man, the Home Secretary.

Rather than shying away from the chamber’s shaky third-term arithmetic, he charged ahead. And he lost. Despite the arrival of Cabinet muscle from far-flung countries, the presence of physically ailing MPs and framing the issue as critical to national security, he not only lost, but he lost badly. It was not just “the usual suspects” behind the defeat, but a broader cross-section of Labour backbench opinion. As such, the result raises very real question marks about Mr Blair ’s ability to herd his party behind his other, necessary reforms.

Why did Mr Blair insist on sticking to 90 days for the time a terrorist suspect could be held without charge? Why reject the advice of Charles Clarke, who four times in recent weeks dropped heavy hints that he believed 90 days to be out of reach? Senior police officers had indicated to The Times that they would be happy with a lesser period; why did the Prime Minister risk his reputation and his reform programme on such a spurious figure? He would surely have won approval for 42 days and would then have been able to push on with other business. This paper has consistently argued that around 30 days was appropriate — with the provision for an extension in extraordinary cases. In that sense, we have had a victory, but the Government has suffered a profound loss of authority. So why did he do it? The answer is politics.

Political romantics yearn for the days of the primacy of the Commons but, in truth, when votes are close the motives of all sides can become grubby. And so it has been with the Terrorism Bill. Earlier this week we warned the Conservatives not to put the chance of drawing prime ministerial blood above domestic security. Michael Howard has stuck rigidly — and successfully — to 28 days and, in so doing, placed his party at odds with many of his own voters. But Mr Blair is no stranger to playing politics. As his soundbite of the day suggested — “Sometimes it's better to lose and do the right thing than win and do the wrong thing” — victory was not necessarily everything he wanted from the vote. Staging a show trial of strength suited Mr Blair. With his unerring ability to scent public opinion, he thought he could emerge triumphant in victory or principled in defeat.

Mr Blair may indeed win praise from 7/7 victims for “doing the right thing”. But his keenness to press the issue may also be a sign of weakness, an indication that he felt the need for a dramatic gesture to recapture authority or mask a defeat. Either way, the result is a serious blow. If he cannot mobilise a 66-seat majority on national security, he is likely to struggle to do so on plans to give the private sector more say in the health and education systems, and to review welfare spending. Mr Blair has looked invincible for the past eight years. But after yesterday, he no longer walks on water. From today he must at least prove that he can walk the line.