Read more columns by Frances Gibb

Gordon Brown may have bought off the 10p tax revolt - but his move simultaneously gave sustinence to the worm of another rebellion: that growing fatter by the day over the proposed 42-day period for detaining terrorist suspects without charge.

MPs in revolt now believe that the Prime Minister is weakened - and having given way once, will find it harder to fight his corner a second time.

It is not just the concession of cash payments for pensioners that has dealt the 42-day plan a body blow. This week the former Attorney-General, Lord Goldsmith, QC, delivered a scathing assault on the plans, giving warning that they would be seized on by misguided young Muslims as a justification to "take up arms".

The former Attorney, in language stronger than any used during his time in office, told MPs that the principle at stake was "enormously important", adding that if still in government, he would vote against.

"When [dealing with] something as significant as individual liberty - which has been part of this country's background - you don't turn that away unless you are sure it is necessary; and not just because you think it might be desirable in the future."

The proposal, he added, was "counter-productive": "it sends a message, particularly to Muslim communities, that we've got a down on them, that this is another attack on them. And I have a worry that some misguided young men will see this as a justification for taking up arms, as they see it, against us." The plans for the extension from the present 28 days were not necessary or proportionate and did not contain adequate safeguards.

The proposed safeguard, in which MPs and peers would be asked to back the Home Secretary's decision to increase the limit, he dismissed as "not a great deal of a safeguard", adding: "Are you going to ask Parliament simply to trust the Secretary of State?" He would be very worried, he said, of questions of individual liberty being decided on party lines.

Lord Goldsmith, who was giving evidence to the MPs sitting on the Counter-Terrorism Bill's committee stage (under a new procedure in which evidence is taken before MPs scrutinise clauses), even admitted that had the Government not been defeated in November 2005 over its original proposals for an extension to 90 days, and the issue had come to a vote in the Lords, he would have resigned from the Government.

In summary, the measure is not needed, is disproportionate to the perceived risk - and carries inadequate safeguards. Above all, it was legislating "in case" - rather than to tackle a problem that had arisen.

The former Attorney was not alone. Earlier that day MPs also heard from Sir Ken Macdonald, QC, the Director of Public Prosecutions, and Sue Hemming, the Crown Prosecution Service head of the counter-terrorism unit.

The DPP reaffirmed his view that there was no need for an extension. "Our experience," he told MPs, "is that we have managed comfortably with 28 days and therefore we have not asked for an increase. And although it's possible to set up hypotheticals, anything is possible... the question is, is it remotely likely?"

"From our point of view, as prosecutors, we don't perceive a need for the period of 28 days to be increased."

Earlier this month Sir Ken told The Times that there was a 92 per cent conviction rate in terrorism cases and that the CPS had not "encountered difficulties in charging those we want to charge".

Sue Hemming backed up the case with figures: since the Glasgow bombing nine months ago, not one case had involved holding a suspect without charge for longer than 14 days; and only three cases in all, since the 14-day limit was extended to 28 days in 2006.

The witnesses were persuasive. Earlier the MPs had the other side put to them - from Sir Ian Blair. The Metropolitan Police Commissioner gave warning that the 28-day limit may be breached "sooner rather than later" because of the "escalating threat".

The growth in terror plots, numbers, conspirators involved, their complexity, breadth and depth and different jurisdictions spanned, made this inevitable. He said that suspects "suddenly emerge from left field" and have to be arrested at a very early stage, leaving officers "with huge amounts of investigative work".

"We have reached a point where at 28 days sooner or later, and maybe sooner, something is going to happen to make that insufficient," he told MPs.

"Because of the level of threat, we sometimes make arrests when we have almost no evidential material at all. So we are starting from a place where we are very, very concerned about what these people are going to do but we are not quite sure what it is. And this leads to some of the most serious charges."

Who is best placed to judge? Sir Ian insisted his investigating officers were; but Sir Ken could make a strong case - now that prosecutors are involved with charging, and work with the police from the start - that they could make the best call.

"You can have honest opinions on both sides," Sir Ian said. "But prosecutors have to judge whether an investigation turns up sufficient evidence to bring a charge. We are very well-placed."

Not only well-placed, but apparently with the best evidence to make the case - against an extension. With the use of the so-called "threshold test" in terrorism cases, prosecutors can charge on a lower standard of proof - on the expectation that more evidence will be forthcoming to enable a charge on the full criteria to be made. "Our judgment," Sir Ken said, "is that the threshold test is sufficient to allow us the scope to charge even in the most extraordinarily complex terrorist offences."

The vote is not due until May or even June. But backbenchers now say it will be impossible for the Government to win.

This is not just a case of lawyers versus the police. Britain's most senior Muslim police officer, Assistant Commissioner Tarique Ghaffur, has admitted that he is opposed; and the former West Midlands Chief Constable, Geoffrey Dear, now Lord Dear, has dismissed the proposals as "flawed".

Above all, he argues, they will lose hearts and minds abroad and among minority groups in Britain - handing a propaganda coup to the terrorists. "Have we not learnt from the failed internment policy in Northern Ireland that miscarriages of justice - real and imagined - are the best way to recruit new terrorist suspects?"

Times Recommends

• Legal school 'earned £1m by exploiting bar course'
• Axe may fall on 5,000 more City lawyers
• Prince Charles named in £81m court battle