Jacqui Smith has been grilled by MPs over her plans to increase the length of time terror suspects can be detained without charge.

The Home Secretary has already faced a barrage of criticism since the plans to up the limit from 28 days to 42 were published last week.

Ms Smith went before the Commons' all-party Home Affairs Select Committee as part of her bid to get the new limit through Parliament.

However, it could prove to be Gordon Brown's toughest challenge so far as Prime Minister, as a previous attempt on the issue led to Tony Blair's first defeat in the Commons.

Under the latest proposals Ms Smith will 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 then have to approve the extension.

Granting MPs and peers a role in the process would be an important safeguard, Ms Smith said.

However, the way the system is set up could mean suspects being held for 42 days even if Parliament eventually refuses permission.

Such a scenario could arise if the Home Secretary only decided to extend the limit towards the end of the existing 28 day slot, because Parliament is only required to vote within 30 days.

In that time, a suspect could already have been charged or released without charge.

Labour and Conservative committee members warned Ms Smith she would face a tough time getting the law changes through Parliament.

Tory James Clappison told Ms Smith that the latest proposals for a 42 day limit were "misleading".

And Labour rebel David Winnick, who played a crucial role in the Government's previous defeat on the issue, warned the Home Secretary there were "battles to come".

Mr Clappison said: "I would suggest that it is a flawed process. It's misleading to describe it as a safeguard."

Ms Smith replied: "You can suggest it to me, and I would disagree with you."

Indicating that Parliament's vote may take place after the 42 day point, Mr Winnick said: "It is a cosmetic exercise, Home Secretary."

Ms Smith replied: "If you believe the voice of Parliament has a cosmetic effect on the executive, then I would disagree with you.

"I believe it is a strong measure of Parliamentary pressure and an important safeguard that any home secretary making a decision will need to be aware that they will come under extremely close scrutiny both within the 30 days and subsequently."

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