Tony Blair and his Home Secretary today offered different levels of commitment to controversial new anti-terror plans.

Charles Clarke told MPs that a proposal to extend the detention time for terror suspects from 14 days to three months - which has enraged political opponents, human rights groups and leading lawyers - was not set in stone.

"I completely recognise there is a concern," he said. "Three months is not a God-given amount and that’s why I indicated flexibility to deal with that in the proper way," he said.

However, Mr Clarke's conciliatory address to the cross-party Home Affairs Select Committee stood in contrast to the Prime Minister's bullish defence of the measures expected to be proposed in the Counter-Terrorism Bill.

The Prime Minister told his weekly press briefing at Downing Street that the police had come forward with a "compelling" case for the extra time, and he had no intention of backing down.

"I’m not saying that whatever the police say, we have got to do it," he said.

"But if the police, charged with fighting terrorism in this country, say to me and to the Members of Parliament ‘This is why we need it’, and that case is a good and compelling case - as I find it is - then my duty is to do it, unless someone can come forward with a very good argument why their case is unsound."

Mr Blair stressed that he did not believe there was any real danger that police would use the power indiscriminately: "I don’t agree that the police would simply bang up anybody they wanted to bang up."

The issue is likely to dominate the next session of Parliament until the new year. Analysts say that while Mr Blair can afford to play 'bad cop', it will be up to Mr Clarke to steer the more controversial measures past opponents on the opposite bench and within their own ranks.

The Liberal Democrats have insisted they would oppose the three-month extension which Mark Oaten, the Home Affairs spokesman, described as "giving in to the terrorists."

The Bill, which is due to be published in full for the first time tomorrow, will also make glorifying or indirectly encouraging terrorism an offence carrying up to seven years’ imprisonment.

The Home Secretary agreed that he believed three months was the correct amount of time and pointed out that suspects can be held for many years while investigations continue in certain European countries including France, Spain and Italy.

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