Brown says he won't back down on 42-day terror detention plan despite fears of a Commons defeat

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 tonight 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.

Standing firm: Gordon Brown says he will not back down over plans to detain terrorist suspects for up to 42 days without charge

But Mr Brown made clear today 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 wrote in The Times.

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 at the weekend from former Attorney General Lord Goldsmith, who warned the move would be an attack on the country's 'fundamental freedoms'.

In an article for The Sunday Telegraph, Lord Goldsmith said: 'There can be no mistake that extending the period suspects can be held without charge is a very serious incursion on our fundamental freedoms.'

Rebel Labour MP David Winnick said Lord Goldsmith's comments demonstrated the difficulties the Government would have getting the proposals past the Lords - even if they cleared the Commons.

But Justice Secretary Jack Straw said the 'overwhelming majority' of Labour MPs supported the Government on 42 days.

While acknowledging there were 'anxieties' about the moves, he insisted Lord Goldsmith's criticisms were unjustified given the 'protections' ministers were putting in place.

He added: 'One of the things we are asking those who have reservations about these proposals is to cut through the rhetoric which surrounds the opposition to the proposals.'

Chief Whip Geoff Hoon added: 'I'm confident that we can win that argument.'

Shami Chakrabarti, director of civil liberties group Liberty, called on ministers to spell out its latest measures designed to win over rebels.

She asked: 'If the Government is so confident in its new last-minute proposals, why doesn't it publish them?'

It was reported today that the Council of Europe, a human rights watchdog, is to warn Mr Brown this week that the 42 day limit would be 'excessive'.

In a letter, it would say that the move would also put Britain 'way out of line' with the rest of Europe, according to The Guardian.

John McDonnell MP, another of the Labour rebels, told GMTV today MPs should be allowed to make up their own minds about the 42-day proposal and not be forced into voting along party lines.

He said: 'Gordon Brown has not made his case. We approved a 28-day limit recently and most of us are a bit bemused why we are here again.

'We arrived at 28 days because Parliament thought that was enough. This is a fundamental human right. MPs should be allowed their judgment on this.

'It's the good thing about parliamentary democracy, we can have a debate.

'We should be unfettered by party discipline and have a free vote. Voting against this is not a challenge to the Prime Minister's leadership.

'It's a fundamental issue about human rights, lets keep it away from party politics.'