Last night Gordon Brown faced personal humiliation over his plan to extend detention without charge. In his effort to avoid this fate, he has humiliated Westminster instead.

The most contentious vote of Mr Brown's premiership was won by browbeating MPs on what should have been an issue of conscience, and with thinly disguised bribery when browbeating did not work. He has won a tactical victory that will be presented as proof of his seriousness about national security. Would that it were.

The reality this morning is a prime minister willing to erode ancient freedoms with cataclysmic threats to shore up his position; a government willing to write unworkable legislation for the sake of a hollow political gesture; and MPs on both sides of the aisle demeaned by the worst sort of horse trading for their support. The currency of Parliament is the integrity of its Members. It has seldom been so wilfully devalued.

Mr Brown launched his campaign to let police hold terror suspects for six weeks without charge knowing that senior law officers, including the Director of Public Prosecutions, saw no need for it. He also knew that it would undermine the very freedoms threatened by terrorism, and that it would send a hostile signal to precisely those disaffected communities where hearts and minds must be won over.

Unable to argue his case on its merits, Mr Brown resorted, first, to flattery and arm-twisting. But there were claims last night that discussions had been held about an aid package for Northern Ireland unrelated to terrorism, but possibly worth up to £225 million, in order to win over the nine Democratic Unionist MPs who proved vital to his Commons victory. And it has been confirmed to this newspaper that two potential Labour rebels were promised a tough British stance against Cuba at the next EU foreign ministers' meeting in return for their support, while a third waverer demanded that terror suspects held beyond 28 days and then released would be offered compensation.

The Home Office has duly promised to develop a scheme that would make payments to uncharged suspects for their spent in jail. This concession, even more than the many others promised by the Home Secretary, betrays the Government's abandonment of principle in its pursuit of the 42-day clause.

Hitherto, depriving crime suspects of their liberty for strictly limited periods, whether or not charges are eventually brought, has been accepted as a regrettable but unavoidable part of the price of justice. The Government has now endorsed the fundamentally flawed notion that pre-charge detention is legitimate only when it leads to a prosecution.

In the war of political attrition leading to last night's vote, Mr Brown subordinated one of the most sacred principles of British justice to expediency. He has also squandered what remains of his personal capital, and for a measure that is not only certain to be rejected by the House of Lords but is now so garlanded with checks and balances that it will serve no practical purpose if forced on to the statute book at some later date with the help of the Parliament Act.

The Prime Minister staked his reputation on 42 days to look strong. Instead, he looks weaker. He has failed to win the argument, so has chosen to strike a deal. He has conceded safeguards to sceptics that make the 42-day bark much less effective than its bite. He has demanded that MPs put their principles aside in the interests not even of party loyalty, but loyalty to the Prime Minister himself. And his whips have offered quiet promises for votes. This hollow victory will buy him time in the Westminster village, but at a sad and further cost to Parliament's prestige.