The question of detention without charge for terror suspects will dominate the debate about Labour's ninth counter-terrorism Bill over the coming months. That fact has killed off, unfortunately, the hopes of those of us who wanted a consensual approach to the vital issue of national security.

The Government has decided to continue to push for an extension to pre-charge detention when there is not a shred of evidence that it is necessary. The Home Secretary has admitted she cannot indicate a single case where an extended period of detention would have been needed.

Instead, they tell us they can "envisage a scenario where it might be necessary". Am I taking my legislative responsibilities too seriously when I feel that the envisaging of a variety of nightmare scenarios is not a sound enough basis for pre-emptively changing the law?

I've heard some other pretty odd arguments in favour of extension, not least that Kafeel Ahmed, one of the alleged terror attackers at Glasgow airport, who fell into a coma and then died in hospital, might have required more than 28 days of questioning. This is absurd. Not only had he been clearly caught on camera driving a blazing vehicle into the terminal building – a sound basis for an immediate charge if ever I've seen one – he was in a coma, and hardly in need of detention.

When the debate is coloured by these sort of misguided claims, it makes it all the more clear to me that the Government is scrabbling around for ideas to justify a solution that they have already decided upon. We already have the longest period of pre-charge detention in the Western world – how can they justify extending it on such scant evidence?

This 28-day issue will hijack the debate on the anti-terror Bill, even though opposition parties, civil society organisations and the Government are largely in agreement over its other important provisions.

We must ask why then, when there is so much we could work together on, they have made a bee line for something that is so divisive. Is it because government ministers feel forced to uphold a 12-month old Gordon Brown promise to secure 90-day detention, a promise he made for purely political reasons as he was trying to crowbar his way into Number 10?

I simply will not accept that we should change the law just because of prime ministerial posturing, or that the Home Secretary should become the prisoner of Gordon Brown's machismo. The principle of habeas corpus is too precious to be traded away so lightly.

It is not just our ancient British liberties at stake however. It is also our safety from terrorism. There is no question in my mind that these heady debates have a negative effect on efforts on the ground to stop home-grown radicalisation taking root.

Of course extremists and preachers of hate will seek to radicalise youngsters within their communities whatever we do. But surely we have learnt by now that breathless talk about the "war on terror" or sloppy anti-terror legislation gives them needless additional ammunition with which to sustain their twisted grievances?

It is worse still when the police and security services are dragged into the controversy. Great outreach work done by the police, such as the Metropolitan Police's Muslim Contact Unit, will struggle when Sir Ian Blair, the Met's beleaguered Commissioner, appears on television as the front man for government plans to extend detention. The Association of Chief Police Officers allowed itself to be used as an advocate for the Government the first time we debated extending detention without charge.

And this week the head of MI5 revealed his latest assessment of the terror threat the day before the Government announced new counter-terrorism measures. Such a coincidence is starting to raise eyebrows. Neither the police nor the security services have any long term interest in getting entangled in a political debate which should be played out in Parliament, where it belongs.

All of this distracts attention from the quiet, low key efforts to tackle radicalisation on the ground, and from the many pragmatic measures which both the Government and opposition parties can find agreement on to strengthen the ability of our courts to better prosecute terror suspects.

As the national debate on terrorism matures, our aim should remain steadfast and simple: to protect both our lives and our liberties, and to refuse to accept that one requires the sacrifice of the other. That is why I will continue to vote against any extension to pre-charge detention.

Nick Clegg is a Liberal Democrat leadership candidate