Philip Webster, Political Editor for The Times, said the Government must continue to talk tough even while it is offering a compromise.

"Tony Blair gave a very forceful performance this morning saying that to compromise over this legislation would be to compromise national security. He evoked memories of July 7 and said that if it stops one bomb, or saves one life, the legislation would be worthwhile.

"He said that the 90-day limit had not originated from the Government, but had come directly from the Association of Chief Police Officers (ACPO) just days after the terrorist attacks on London.

"He said that while he would not always give police everything they wanted, he found the case that they put forward to be compelling.

"However, he knows quite rightly that the 90-day custody limit is not going to get through. So at the same time as he was doing the tough stuff, Charles Clarke was having to deal with the reality of the situation.

"If he were to stick to 90 days, he would lose the vote on Wednesday. While the negotiations go on, and while Blair tries to convince the MPs, they are having to prepare a fallback situation.

"There is already an amendment for 28 days, which was tabled by the Labour MP David Winnick. I suspect that later today we will see the Government table another amendment for a middle way. The figure that is going around at present is 42 days.

"There will then be argument over whether it should be 28 or 42 days and, if the Tories say that they couldn't conceive any more than 28 days, Blair can portray them as being soft on terror. Meanwhile, he will have tried to have won back enough Labour backbenchers to ensure that the Bill is passed.

"Tony Blair faced repeated questions today over whether this was symptomatic of his authority waning. He rejected them all, point blank. He said that this was not a vote of confidence in his leadership, and that it was more important than that - it was about public safety.

"When he meets the Parliamentary Labour Party tonight he is likely to tell them that he is not taking this controversial line in order to cause trouble, but because he thinks it is a key platform which will win Labour a fourth term under a new leader. He will tell them that Labour has to show that it has not run out of steam.

"Labour always has a problem over passing what looks like draconian legislation - to go from 14 days to 90 is a massive jump - but Blair can argue that he is completely in tune with the public mood on this. Indeed, this is the first time I can remember his getting into such trouble on an issue where the public is completely on his side."

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