Jacqui Smith, the Home Secretary, today defended controversial new terror laws that may put her on a collision course with Labour rebels and could even see Gordon Brown's first Commons defeat as Prime Minister.

Ms Smith announced last month that she would seek to raise the limit that terror suspects could be detained without charge from 28 days to 42 in a Counter-Terrorism Bill that will be published today.

A recent survey of Labour MPs suggests that enough of them might vote against the Government to defeat it in the Commons. Only 34 votes are needed to defeat the plans and the poll found at least 38 MPs were vowing to oppose moves to extend the existing limits.

The Bill will see ministers return to an issue which led to Tony Blair’s first defeat in the Commons in November 2005, when he sought a 90-day limit.

Appearing on BBC Radio 4's Today programme this morning, Ms Smith said that the Government could not afford to "sit on its hands" in the battle against terrorism.

"It's growing in scale. It's becoming more complicated in nature. People need to intervene earlier because of the way in which it aims to cause mass casualties with no warning," she said.

Asked whether she was proposing legislation to deal with a hypothetical situation, the Home Secretary replied: "We are putting in a provision for if it becomes un-hypothetical."

Under Ms Smith’s proposals, the Home Secretary would be able to immediately extend the limit to 42 days if a joint report by a chief constable and the Director of Public Prosecutions backs the move.

The Commons and the Lords will have to approve the extension within 30 days - although if the Home Secretary only decided to extend the limt towards the end of the existing 28-day slot suspects could be held for longer even if Parliament eventually refused permission.

Last November there were red faces in the Home Office when Lord West, the security minister, said that he was not convinced that more time was necessary.

In a radio interview, Lord West said: “I still need to be fully convinced that we absolutely need more than 28 days.”

Within an hour, after attending a meeting with Mr Brown in No 10, he emerged to issue a hasty U-turn, telling reporters: “My feeling is, yes, we need more than 28 days.”

Today’s Bill is also likely to include other counter-terror measures including moves to allow police to question suspects after they have been charged.

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