Security minister Lord West has warned of future security threats
Security Minister Lord West has been accused of being "reckless" after saying a "great plot" by terrorists was being monitored.
Lord West made his comments in the House of Lords the day after it threw out plans to extend terror detentions.
He said the terrorist threat was "huge" and said: "There is another great plot building up again."
The Tories said it was "reckless in the extreme". The Lib Dems said he risked "blowing the cover" of an inquiry.
Lord West was updating the House of Lords on the government's position earlier, a day after plans to extend pre-charge detention limits to 42 days suffered heavy defeat in the Commons.
He told peers that while some measures had been taken over the past 15 months to make Britain safer "this does not, I'm afraid, mean we are safe".
He said: "The threat is huge. The threat dipped slightly and is now rising again with the context of 'severe', large complex plots, because we unravelled one the damage it caused to al-Qaeda actually faded slightly.
"They are now building up again. There is another great plot building up again and we are monitoring this."
The BBC's security correspondent Frank Gardner said counter-terrorism sources were "slightly baffled" at Lord West's comments.
He said while there were continuing investigations, no-one he had spoken to was aware of any one "great plot building up again".
Later shadow home secretary Dominic Grieve described Lord West's comments as "reckless in the extreme" and could jeopardise police efforts to "strike a balance between early arrest during a developing terrorist conspiracy in order to protect the public, and waiting long enough to ensure there is enough evidence to secure a conviction."
Conservative MP Patrick Mercer, a former colonel and member of the Home Affairs Select Committee, told the BBC: "I fear the cat has been let out of the bag ... it's not a great idea to tell your potential victims that you are onto them and you are watching them."
Last year, Lord West told the BBC he had yet to be convinced of the need to extend the current 28-day detention limit. He then denied being forced to change his mind when, within an hour and after a visit to Downing Street, he said he was actually convinced.
The former head of the Royal Navy, who was made a minister as part of Gordon Brown's "government of all the talents", said he was just a "simple sailor" who had not chosen his words well.
On Tuesday, Liberal Democrat home affairs spokesman Chris Huhne said Lord West needed to "tame his tongue" and said his comments on the terrorist plot risked blowing the cover of a counter-terrorist investigation.
But government sources say Lord West's comments reflected Home Secretary Jacqui Smith's comments that the threat was "at the severe end of severe" and said he had revealed nothing about what, who or where was being investigated.
During the debate in the Lords several peers criticised Home Secretary Jacqui Smith's implication, in her statement after Monday's vote, that 42-day opponents took security "lightly".
Lord West said she had not meant the comments "in exactly the way they have been taken", to protests from peers who said he should disassociate himself from the comments which they said had caused "enormous offence".
He added that he thought "we'd done rather better" during the debate about the bid to extend terror detention limits to 42 days and was "horrified" at the scale of the government's defeat - the measure was thrown out by 309 votes to 118.
Within two hours of the vote, the government announced it was dropping the 42-day proposal from the Counter-Terrorism Bill.
But the home secretary said she had written the plan into a separate one-page bill which could be pushed through Parliament quickly in the case of a national emergency.
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