The 28-day limit on detaining terror suspects without charge will need to be breached sooner rather than later because of the escalating threat of attack, Sir Ian Blair, the Metropolitan Police Commissioner, warned today.

The growth in the number of terror plots, numbers, conspirators involved, their complexity, breadth and depth and different jurisdictions spanned, made this inevitable, he said.

Sir Ian, giving evidence to MPs at the committee stage of the Counter-Terrorism Bill, 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 we feel sooner or later - and maybe sooner - something is going to happen to make that insufficient,” he said. He added: “We have reached a point with 28 days where we are pushing at that.”

Sir Ian and Bob Quick, the Assistant Commissioner and Scotland Yard’s anti-terror chief, told MPs that 15 terror plots have been foiled by police since the July 7 attacks.

They added that the UK - when set against the United States, France, Spain, German and Italy - was at particular risk of terrorist attack. “We appear to face the most radical and escalating threat of these countries in terms of the number of people involved and plots going on and that are happening now.”

The Commissioner added that the an extension of the time that suspects could be held was needed all the more because the use of intercept evidence - now under consideration - was “not going to be available for many years to come”.

The Government is facing a back-bench revolt over the plans to extent the 28-day limit to 42 days with even some ministers not convinced.

In a second difficulty for the Government, the High Court tomorrow is expected to rule against it over powers to freeze suspects’ bank accounts.

Under the provisions, suspects need a licence from the Treasury before they can obtain legal advice about their status as a “designated” suspect whose assets can be frozen.

Sir Ian said that he was not wedded to the figure of 42 days: his position was that the 28-day limit needed extending and it was for Parliament to decide the figure.

The old limit of 14 days had come under the same pressure in 2004; and in 2006 the present 28-day limit was under pressure and “we did detain for 27 days before charge,” he said.

Under a new procedure, only used once before, the committee of MPs is hearing evidence on the Bill before examing each of its clauses.

Sir Ian added that since the 7/7 London bomb attacks, he estimated that a total of 12 plots - all likely to have involved loss of life - had been foiled. Mr Quick put the figure at 15.

In response to the view of the Director of Public Prosecutions, Sir Ken Macdonald, QC, that the 28-day limit was sufficient, Sir Ian said that it was for the police to conduct the investigations, not the prosecutors, who did not necessarily know the difficulties involved.

“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.”

Some of the strains and pressure on investigating officers were not necessary entirely visible to the Crown Prosecution Service and the prosecution team, he said.

“There are practicalities - where officers have to trail all around the world, with very extreme and tight deadlines, working with high technology teams breaking encryption codes ....that places real pressure on people to deliver the evidence.

Sir Ian said that the threat now faced differed from that posed by the IRA in early and mid-90s in four respects: the terrorists then gave warnings; they did not want to die; they did not want to cause mass casualties and they were heavily penetrated by British intelligence.

Nor did they make much use of mobile phones, which at that time were only just coming in, or of computers.

Now, terrorists formed a very loose network but communicated through the internet from around the world and from different jurisdictions, he said.

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