The home secretary has lowered her ambitions to increase the time police can hold terrorism suspects without charge.

The current limit is 28 days and Jacqui Smith said it should be increased to 42 days in "exceptional circumstances".

But ministers have previously expressed a preference for a 56 or 58-day limit.

In 2005, Tony Blair suffered his first Commons defeat as prime minister when he tried to push through legislation to increase the limit to 90 days.

And as the debate returns to Parliament under Gordon Brown, the prime minister has said there is a "growing weight" of support for extending limits.

However, the plans are opposed by the Conservatives and Liberal Democrats, as well as civil liberties groups.

Sir Ken MacDonald, the director of public prosecutions, and former attorney general Lord Goldsmith have also raised concerns over the need to extend detention periods.

MPs on the Commons home affairs committee - also thought to be sceptical - will meet later to discuss the issue.

'Compelling'

Under the measures, MPs and peers would be given a vote to approve emergency powers to give police more time to question suspects.

If an arrest was made during summer recess, Smith said Parliament may be recalled to vote on an extension.

"We are proposing that where there is a compelling operational need, the home secretary can extend the operational limit that a terrorist can be held for up to a maximum of 42 days," she said.

"To emphasise that the higher limit is exceptional, it must be approved by Parliament within 30 days."

After two months, the police would be required to renew the limit.

Pointing to support from Lord Carlile, the government's reviewer of terrorism legislation, Smith added: "This isn't about win-win.

"It is about legislating now for a risk that I am clear does exist, chief constables are clear exist and the independent reviewer of terrorism legislation is also clear exists."

'Evidence'

The Conservatives have warned that the move could be counter-productive and shadow home secretary David Davis accused the government of "bamboozling".

"When the government start bamboozling like this, you know the game is up," he said.

"They are making a proposal for something they still have not proved necessary. They have lost the argument to further extend pre-charge detention beyond 28 days again and again."

He also criticised plans to allow MPs and peers to vote on individual cases, saying: "Parliament is certainly not the appropriate mechanism to hear the details of individual cases because of the risk of prejudicing a trial."

Meanwhile, the Liberal Democrats called on the government to make a Commons statement on the issue.

"The government is tying itself up in knots in an ever-more desperate attempt to sweeten the bitter pill of this proposal for its own backbenchers," said Lib Dem home affairs spokesman Nick Clegg.

Claiming that the decision to extend beyond 28 days would be taken by the home secretary and would only be put to a vote after 42 days, he said: "Frankly, what the government is now proposing is not easy to understand.

"The fundamental liberties and safeguards enjoyed by generations of British citizens cannot become the plaything in ever more complex horse-trading between the home secretary and her backbenchers."

Consensus

Downing Street said the government wanted to reach a consensus on the issue.

"The key thing in relation to today's proposal is not to get too fixated on the number [of days] but more to focus on the safeguards around the circumstances where it might be necessary to detain someone," said the prime minister's spokesman.

"We have listened to the concerns that have been expressed. We have always said we want to do this in a consensual way."

However, Liberty director Shami Chakrabarti accused the government of abandoning attempts reach an agreement.

"It seems more like politics than policy-making to me. I think it is a real mistake as I've never seen such good will in the House of Commons," she said.

"I believed there was a unique opportunity to end the arms race on terror laws, to actually achieve something that would last beyond just one government."