The Tories would "almost certainly" reverse Gordon Brown's 42-day terror detention plans if they were in power, the shadow home secretary said today.

The Tories would "almost certainly" reverse Gordon Brown's 42-day terror detention plans if they were in power, the shadow home secretary said today.

Frantic last-ditch negotiations were under way today as the vote on extending the detention period from the current 28 days approaches its moment of truth.

David Davis told GMTV that even if the Government wins the vote the battle will go on.

When asked why the Tories oppose the plans, he said: "What characterises Britain in the world at large is liberty under the law. We have for centuries been the freest country in the world, and now we are actually the other way round," he said.

Secondly, he said, the measures would not help against terror in the way they are intended because they would make the community, particularly the Muslim community, feel picked on and lead to difficulties in gathering intelligence.

The Crown Prosecution Service does not believe an extension is necessary, he added.

Following his comments, Tory leader David Cameron accused the Prime Minister of "ineffective authoritarianism" by proposing to detain terror suspects for 42 days without charge.

In a fierce Commons clash just hours before the vote on the issue, Mr Cameron branded the Government's plans "unworkable" and a "symbolic assault on liberty that is unnecessary".

But Mr Brown insisted the change was needed to tackle increasingly sophisticated terrorism and accused the Tories of "opposition for opposition's sake".

He said he would rather act in "calmness" now than try to bring in "panic" measures at a time of emergency later.

The row over today's vote on counter-terror measures dominated the 30-minute question time session just ahead of today's key debate.

Mr Cameron warned the extension to 42 days risked being counter productive and cautioned the Prime Minister against "trashing" the nation's hard won civil liberties.

But Mr Brown, citing police backing, insisted it was his duty to do not what was popular but what was right and necessary for the security of the UK.

In his earlier TV interview, Mr Davis said that if the Tories were in power they would look at the information, but added: "I think it's almost certain we would reverse it."

Instead, they would focus on using intercepted and bugging evidence in court, plea bargaining and allowing suspects to be interviewed after being charged, as well as a "much harder" approach on dealing with people who radicalise young men and women.

"We must enforce the law," Mr Davis said. "Our freedom, our justice system, our sense of justice in our country is our strongest weapon."

As the Commons braced for a crunch showdown, government whips were said to be offering potential rebels their "heart's desire" in return for supporting the controversial proposals.

Home Office minister Tony McNulty also signalled a fresh concession by suggesting that terror suspects who were eventually freed without charge could be paid compensation.

However, tonight's vote was still on a knife edge, with Labour rebels insisting their numbers were holding strong.

The DUP - whose nine MPs could swing the result - will meet this morning to discuss their course of action. There has been speculation that they are on the verge of doing a deal to back the extension - which would almost certainly prevent a Government defeat.

But campaigning group Liberty launched a final bid to force the issue last night, accusing ministers of "sexing" up claims that police had already been "up against the buffers" of the current 28-day limit.

It said a lawyer involved in the Heathrow airline plot case had dismissed claims that detectives needed until the final day to gather crucial evidence against two suspects.

Instead, the key material was obtained within 12 days of the individuals' arrest, according to Liberty.

Downing Street admitted yesterday that there was still "more to do" to ensure victory in the vote, and the Prime Minister is believed to have been calling waverers personally.

In a sign of how tight the situation is, Foreign Secretary David Miliband has cut short a visit to Israel to attend the crucial divisions in Westminster.

Mr McNulty indicated last night that the Government was considering adopting an amendment to the Counter-Terrorism Bill that would compensate suspects who were detained for longer than 28 days but not subsequently charged.

He told BBC2's Newsnight the idea seemed a "perfectly reasonable proposition".

"The Home Secretary (Jacqui Smith) will look at it and she will report back to the House," Mr McNulty added.

Labour rebels believe the Government wants to secure as big a majority as possible in order to strengthen its hand ahead of a potential confrontation with the House of Lords - where opposition to the measure is even more entrenched.

Ministers argue that following a series of concessions last week it has now built in sufficient safeguards to allay civil liberties concerns.

They insist the 42-day maximum is purely a reserve power which can only be activated by a vote of both Houses of Parliament.

Recent opinion polls have suggested that between two thirds and three quarters of the public is behind the extension.

However, critics remain unconvinced. The joint Commons and Lords Human Rights Committee wrote to Ms Smith last night warning that the Government's safeguards still did not provide effective protection against arbitrary detention.

"We do not doubt the seriousness of the threat from terrorism," said the committee chairman, Labour MP Andrew Dismore.

"But it is hard to see the necessity for a still more draconian power when it has not been necessary in the last year to use the existing exceptional power to detain for more than 14 days."