The man has gone but his spirit lives on. Tony Blair's determination to turn the war on terror into a permanent undeclared state of emergency in Britain, where the "rules of the game have changed", ended in defeat two years ago when he failed to raise the limit on detention of terrorist suspects without charge from 14 to 90 days. With a parliamentary compromise of 28 days in the bag - already far longer than any other state in the western world - it might have been expected that his successor would be content to leave well alone. Not a bit of it. Gordon Brown is back for more, pressing the case this week for the right to imprison people without charge for 56 days, or however close to that figure he can manage.

What started a generation ago as a two-day limit on detention without charge, as exists for American citizens in the US, was fixed at seven days in 2000; ratcheted up to 14 in 2003; raised again to 28 in 2006; and is now heading for two months of effective internment. The arbitrariness of this ratcheting-up is obvious: in spite of the fact that we're talking about the country's most basic civil liberties, it has clearly been a matter of think of a number and double it.

Perhaps that's not so surprising, as nobody has been able to offer any evidence whatever that police investigations have been undermined by having to release or charge a suspect within four weeks. Indeed, despite much talk of the growing complexity of terror cases, the home secretary, Jacqui Smith, has now conceded that no circumstance had yet arisen where "it has been necessary up to this point to go beyond 28 days". To all intents and purposes, the police and government case is simply that it might be a useful precaution for the future - or even a helpful "disruptive mechanism" when there is no real chance of a charge. And now that the human rights organisation Liberty and the opposition parties have offered an alternative of post-charge questioning (which carries its own dangers), ministers have pocketed the concession and pressed on regardless with their longer detention plans.

Most shamefully, it's widely acknowledged in Westminster that a key motivation for this latest assault on long-established rights and freedoms is Brown's determination to wrong-foot the Tories tactically and portray them as soft on terror. Given the trauma endured by Muslim prisoners - because of course that's who we're talking about, at least for now - locked up for weeks and then released without charge, it might seem to be a bit of rather costly political point-scoring. But then it follows the chaotic and counterproductive saga of prisoners locked up indefinitely without charge in Belmarsh in the same spirit, and the draconian control orders that the law lords last week called to be marginally watered down: the regime of 18-hour curfews, bans on phone, internet and personal contacts imposed on suspects without charge, or access to any evidence against them.

This week's Queen's speech proposal to press ahead with yet another extension of the power to lock people up without charge came hard on the heels of the lurid warm-up act from the new MI5 director general, Jonathan Evans, whose organisation will have doubled in size by 2011. In an inflammatory and highly ideological speech, he warned that 15-year-olds were being "groomed" for terror and that the number "we are seeing involved in terrorist-related activity" was now 2,000 - followed by the bizarre rider that "there are as many again that we don't yet know of".

Both figures should probably be taken with a pinch of salt, in the light of the British security and intelligence agencies' erratic record with intelligence over the years. Of course, there are underground jihadist elements prepared to stage violent attacks in Britain, as has already been brutally demonstrated. But the real surprise is how few attempts there have been since Blair joined George Bush's war on terror: one serious atrocity, two bungled outrages, and a series of unsuccessful plots of varying credibility does not even begin to match the scale of the IRA campaign of the previous three decades. Naturally the security services would like to claim credit for that, but given how easy it is to get hold of guns in Britain or bomb soft civilian or political targets, it is obvious that the numbers seriously committed to launching such attacks are fewer than Evans's figures suggest.

What is clear is that the assault on basic liberties represented by repeated extensions of pre-charge detention and control orders is out of all proportion to the reality of what is actually taking place. It also makes a mockery of the government's claim to be defending our freedom and way of life against al-Qaida, when in fact it is trading away another bit of freedom for every bomb attack or terror scare. The danger is not only that we lose valuable liberties, but that we create more terrorists in the process, by further alienating Muslim youth already radicalised by British and American aggression in the Muslim world. That was the message of last week's highly effective Channel 4 drama Britz, denounced by a government spokesman for ignoring the views of "moderate Muslims".

In fact, as polling shows, the kind of concerns expressed in Peter Kosminsky's film - about the impact of anti-terror laws and western foreign policy - do reflect mainstream British Muslim opinion, and it is the government which is failing to face up to that. In an increasingly Islamophobic climate, the support given by some ministers to those in the media and rightwing thinktanks arguing against engagement with representative organisations like the Muslim Council of Britain and non-violent Islamist groups is bound to backfire. Last week, for example, the Blairite cabinet minister Hazel Blears championed the former Islamist Ed Husain - who follows the neocon line on "Islamofascism" and criticises MI5's boss for "pussyfooting around" - as a "new voice" who "understands what needs to be done".

Picking people who are off the map of Muslim opinion to speak on British Muslims' behalf is a dangerous game that will do nothing to increase public safety. The biggest contribution this government could make to reduce the threat of jihadist terror attacks in Britain would be to withdraw occupation troops from Iraq and Afghanistan and end its support for dictatorships in the Muslim world. In the meantime, it could make a start by letting Muslims speak for themselves, instead of locking more of them up for longer without charge.

s.milne@theguardian.com