The government was last night warned by the opposition it will face a ferocious battle to extend the time terror suspects can be held without charge from 28 to 42 days.

MPs gave the Counter-Terrorism Bill an unopposed second reading after a stormy six- hour debate, but the Tories, Liberal Democrats and a number of senior Labour back benchers served notice that they will oppose the controversial detention proposals tooth and nail at a later stage.

Gordon Brown said: "There is no proposal for a blanket extension to 42 days. That is a myth and that is wrong."

Home Secretary Jacqui Smith found herself under fire from all sides as she insisted 42 days would be a reserve power "not to be used lightly" but essential to counter terrorist cells. "As the threat from terrorism evolves, so our laws must adapt to remain effective," she argued.

Shadow home secretary David Davis said: "There is not one shred of evidence for extension beyond 28 days."

He said: "The proposal within this Bill to extend detention without charge for up to 42 days gives up essential liberties without delivering any additional even temporary safety."

Ms Smith insisted she had moved "a considerable way". "We are not now proposing a permanent, automatic or immediate extension to pre-charge detention beyond the maximum limit of 28 days.

"We are proposing a reserve power that would mean that a higher limit could only become available if there was a clear and exceptional operational need, supported by the police and the Crown Prosecution Service, and approved by the Home Secretary."

She said this would be subject to parliamentary approval within 30 days and whether an individual was held will be a decision for a senior judge.

With both the LibDems and Tories supporting key aspects of the Bill, apart from the detention proposals, the second reading was nodded through without a vote, with the real battle to come later.

The plans to extend the limit could also face a court challenge from the Equality and Human Rights Commission.