Three years ago, Tony Blair backed a Bill that would have increased from 14 to 90 the number of days that police could hold terror suspects before charging them. The supporting evidence was weak. Even with memories of the July 7 bombings just four months old, Mr Blair fell to his first Commons defeat. Yesterday the Home Secretary tried again. Her proposal was much less ambitious, but the argument for it is as flimsy as before and the dangers it poses to civil liberties are almost as great. The 42-day plan, like its predecessor, risks and deserves defeat.

The threat of terrorism remains real and acute. Multiple successes by the authorities in thwarting extremist conspiracies do not make it any less so, and as a response to this threat the new Counter-Terrorism Bill is not the craven legislative machismo that many Conservatives and Liberal Democrats would have voters believe. Its provisions for DNA sampling and post-charge questioning of terror suspects are welcome. It is also better to legislate at leisure than in panic - or, as Jacqui Smith put it in a soundbite of tortured syntax and logic: “We are bringing in a position for if [the need for longer detention] becomes unhypothetical.” But her pre-charge detention plan as written is vaguely worded, morally dangerous and unnecessary.

Police currently have four weeks from the date of arrest to build cases against terror suspects. Ministers claim that extra time is needed because of the sheer complexity of the conspiracies they face, many conducted in foreign languages and with computer encryption. Such challenges should not be understated. But there have been no cases to date in which charges might have been brought in six weeks that were not brought in four or fewer, and only 11 suspects have been held without charge for longer than 14 days since the limit was raised to 28 days in 2006.

The Government's choice, Ms Smith asserts, was between sitting on its hands and acting as it has. Yet the resulting Bill fails to define the circumstances in which police could ask a judge to allow extended detention, beyond the catch-all phrase “exceptional operational need”. The Bill also requires parliamentary approval of any extension, but only within 30 days. By this time a suspect could already have been detained without charge for the maximum 42 days. The upshot is a proposal for which there is no proven need, with no real parliamentary oversight but every risk of allowing the detention of innocent people without charge for up to six weeks.

This is not just inimical to justice. It may also prove counter-productive in practical terms. The critical factor in counter-terrorism is accurate, timely intelligence from within the disaffected minorities among whom extremism takes root. Few measures are more likely to alienate the sources of such intelligence than extended detention. This newspaper has defended the idea of extensions in exceptional circumstances before, and could again, but not as currently presented.

Last November, in his capacity as the anti-terrorism czar, Lord West of Spithead told the BBC that he was not persuaded by the arguments for extended detention without charge. Barely an hour later, after meeting the Prime Minister, he said he was persuaded after all. If he was told of specific cases where two extra weeks might have meant important convictions, Parliament deserves to know. If not, the conclusion must be that this plan has more to do with politics than security. It has no place on the statute book.