His defiant comments came at the start of a concerted push to win over Labour rebels before a showdown vote later this month.

Home Secretary Jacqui Smith will appeal to Labour MPs to fall in behind the anti-terror plans when she addresses the Parliamentary Labour Party (PLP).

With up to 50 Labour MPs ready to vote against the plans, Ms Smith has signalled there could be further concessions on when and how the new police powers will be used.

But Mr Brown made clear he would not give ground on the central point.

"I will stick to the principles I have set out and do the right thing," he said.

His stance is reminiscent of Tony Blair's before he suffered his first Commons defeat in trying to extend the detention-without-charge period to 90 days in 2005.

Parliament ultimately approved the existing 28-day limit on that occasion.

But this month's vote comes at a more difficult time for Mr Brown after his embarrassing U-turn over the 10p tax row and a run of disastrous election results.

The 42-day proposals came under renewed attack from former Attorney General Lord Goldsmith, who warned the move would be an attack on the country's "fundamental freedoms".