A "significant" terror plot in four cities was stopped and charges are likely within days after 12 men were arrested, the independent reviewer of counter-terrorism powers has said.
Lord Carlile, who observed part of the surveillance operation on the suspects, said they were under observation for a long time and it was "very possible that people may well be charged and prosecuted" over the next few days.
The 12 suspects, aged between 17 and 28 and at least five of Bangladeshi origin, were held during early morning raids by unarmed officers in London, Cardiff, Stoke and Birmingham on Monday.
It is understood the men, a mixture of foreign nationals and some who were born in Britain, were under surveillance for months.
Lord Carlile said: "The briefing I have had shows there to be allegations of a significant plot in the cities mentioned."
Giving evidence to the Commons Home Affairs Select Committee, Lord Carlile continued: "The gestation period for the arrests has been long. 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. I believe that it is very possible that people may well be charged and prosecuted. 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."
The threat to the UK's security was "indefinite", Security Minister Baroness Neville-Jones said.
Speaking before the meeting of the National Security Council, she said: "I would say it was an indefinite duration, yes, and of course this country also faces a threat from Irish terrorists as well."
Giving evidence to the MPs before Lord Carlile, she continued: "It's very clear that these were important arrests and, as the police said, necessary for the safety of the nation, with which I think the security service undoubtedly agrees and we should take that at face value."
