Terror-arrests charges 'likely within days'

But they were also told of serious concerns about the future effectiveness of powers to impose "control orders" on suspects if nothing is proved.

Police were still searching homes in London, Cardiff, Birmingham and Cobridge, near Stoke on Trent, last night, following Monday's dawn arrests of 12 men "on suspicion of the commission, preparation or instigation of an act of terrorism in Britain".

The suspects are aged between 17 and 28 and at least five are of Bangladeshi origin, it has emerged. Nothing has yet been said about what they are accused of but Scotland Yard Assistant Commissioner John Yates said yesterday that, based on information he had seen, the arrests were "absolutely necessary in order to keep the public safe".

And in evidence to the Commons Home Affairs Select Committee, a government legal adviser on counter-terrorism legislation, Lord Carlile of Berriew, said he knew something of the long investigation which had led to the arrests and "the briefing I have had shows there to be allegations of a significant plot in the cities mentioned".

He said: "I was aware of an operation some time ago which led to these arrests. On one occasion I was able to observe, literally observe, some of it occurring.

"The gestation period for the arrests has been long. I happen to know the Crown Prosecution Service has been involved for some time in this operation and I would expect significant activity over the next few days."

But questions have been raised by the revelation that the police who carried out the raids on Monday were unarmed.

In April 2009, a similar swoop, involving 12 arrests across northern England, led to no charges, although the authorities continued to claim they had thwarted an Al Qaida bomb plot based in Manchester. In June 2008, five arrests in the Stoke area, on similar suspicions, also led to no charges.

Similarly inconclusive investigations have led to eight unconvicted terror suspects, all British, living under restrictions on movement, meetings and personal communications, it was revealed last week by the Home Secretary.

The powers behind these restrictions are due to be renewed on February 10. But the Liberal Democrats went into the General Election with a pledge to scrap control orders and political problems surrounding the review of the legislation have become so fraught that David Cameron was reported last month to have warned colleagues: "We are heading for a car crash."

Although a Liberal Democrat himself, Lord Carlile expressed concern about the "political deadlock" yesterday.

He said everyone understood there was a small cohort of suspected terrorists who could not be prosecuted and "against whom some protection is required".

He said: "The issue is how we make that protection work.

"If there is to be a change, and I'm quite happy about some dilution, it should be a dilution based on the knowledge and success of the system which has now existed for five years.

"There are a couple of key ingredients that are absolutely essential.

"If we do not have curfews, and if we do not have limitations on meetings and the use of the internet, then we might as well not have them at all.

"If we don't have them at all, then in my judgement there will be terrorists walking the streets who present a great danger to the public. That would involve playing political games with public safety.

"My life would have been so much easier if I had happened to agree with all my Liberal Democrat friends who are hotly opposed to control orders."