The Home Secretary has failed to make the case for this suspension of our liberties

The prevention of Terrorism Bill, which the Government is attempting to bludgeon through Parliament, is just as illiberal as the legislation it is designed to replace.

The prevention of Terrorism Bill, which the Government is attempting to bludgeon through Parliament, is just as illiberal as the legislation it is designed to replace. If it becomes law, we will be faced with a situation whereby a person in Britain can be detained solely by the command of the Home Secretary. That person will not be allowed to know on what grounds he or she is being held, or for how long. By entrenching the principle of arbitrary detention, the Bill outlined by Charles Clarke in the House of Commons yesterday will undermine one of our fundamental civil liberties: the right to a fair and open trial. That is the stark case against the Bill. The Home Secretary has failed to answer it.

When the leaders of the two main opposition parties were invited to Downing Street last Friday, there were rumours that the Government was preparing to amend its proposals to meet widespread concerns about the infringement of civil liberties. That has turned out to be a vain hope. The Home Secretary yesterday outlined his programme with only the faintest of gestures towards the primacy of the judiciary. The court orders that will be imposed on terror suspects - a range of restrictions such as tagging, curfews and a ban on internet use - will be subject to judicial review. Judges will also be asked to sanction the Home Secretary's decision to impose what amounts to a sentence of house arrest. But neither of these hastily introduced safeguards, as Mark Oaten, the Liberal Democrat home affairs spokesman, pointed out, answers the central objection: that it is the Home Secretary who will make the initial decision, rather than a judge.

Mr Clarke attempted to sweeten the pill by arguing that when the 11 terror suspects are released from Belmarsh and other institutions next month he will not immediately use the most draconian tool made available to him by the Bill. The power to order "house arrest" will not be invoked, since the security services have convinced him they can keep tabs on the suspects with control orders. Indeed, he points out that the use of "house arrest" would require a derogation from the European Convention on Human Rights, and thus secondary legislation. Parliament would effectively be asked to vote on whether Mr Clarke's action is justified. But, again, this does not address the fact that judges will not be initiating the process.

The only sensible way to proceed with regard to those the Government accuses of plotting terror attacks is to bring them to trial. The Government must devise legislation that will permit the use of phone-tap evidence in a court - a measure that both main opposition parties have recommended. This is not to be undertaken without rigid controls. To protect the security sources, intercept evidence ought to be vetted by separate judges in a pre-trial hearing. This is the only way to protect the public and, at the same time, preserve our civil liberties.

An unpleasant whiff of party politics permeates this debate. Some suspect that the Government would not be overly concerned if this Bill failed in the House of Lords because it would allow Labour to portray the Tories as "soft on terror". Whatever the motivations of the Tories in this affair, they have adopted a laudable position. And it is utterly disgraceful for Labour to insinuate that everyone who opposes its proposals is unconcerned about the terrorist threat to Britain.

At the nub of the debate is the question of just how grave that threat is. The Government argues that the sprawling terrorist entity known as al-Qa'ida represents a danger of a different order to anything Britain has ever faced. We disagree. We are facing a substantial threat from terrorism, and we expect our security services to play a vital, and unprecedented, role in protecting us in coming years. But we do not think the scale of the threat justifies the suspension of the right to trial. That was the unequivocal judgement delivered two months ago by the law lords on the Government's previous anti-terror legislation. It is our judgement on the Bill that is presently before the House.