The home secretary and the prime minister have defended their plans for a sweeping extension of anti-terror powers.

On Wednesday Charles Clarke unveiled new measures that could see foreign and British terror suspects placed under house arrest without trial.

Civil liberties campaigners and legal experts have criticised a further extension of the state's powers.

But Tony Blair warned that Britain faced a serious terrorist threat that had to be dealt with.

"I pay great attention to the civil liberties of the country," he said.

"But on the other hand, it is also right that there is a new form of global terrorism in our country, in every other European country and most countries around the world.

"They will cause death and destruction on an unlimited scale and they will and are trying to organise such terrorist activity in our own country.

"I just hope people get this in perspective."

And the home secretary also mounted a robust defence of the plans.

Clarke said that while he would be taking "substantial powers." they would only be used "in a very small number of cases".

"But the powers are necessary because we do need to protect ourselves against those who would seek to destroy every element of our society, whether it is parliament, government, freedom of speech, the law, whatever it may be," he told the BBC.

"And that is their aim and ambition. It is a very heavy burden, and of course there is controversy about it, but I think we would be very, very foolish indeed not to recognise that there are individuals and organisations which have as their ambition to destroy everything we all stand for."

The home secretary said that those who had criticised his proposals were "perfectly reasonable".

"On the other hand I would invite them also to put themselves in the position that I am in, to weigh the wrong which is being done to a tradition in history of the primacy of law in these circumstances versus the wrong that would be done were any of these terrorist organisations to succeed in their ambitions to hundreds, maybe thousands even of people who are utterly innocent of anything.

"That is the balance I have to make."

Cabinet

On Thursday morning the Cabinet met and according to the Number 10 official spokesman "congratulated" Clarke for "handling what is a very difficult issue".

In the absence of Tony Blair, who was in Davos, deputy prime minister John Prescott chaired the session which heard a report from Clarke on his statement to the Commons.

The spokesman said that Cabinet recognised the decision would provoke widespread debate across the country but commended the home secretary for his work on the area.