The limit for holding terror suspects without charge should be increased, the security minister said today.
Lord West said the complexity of intelligence and scale of the threat posed meant the current 28-day limit should be increased.
His comments came a day after Ken Jones, head of the Association of Chief Police Officers, said terror suspects should be held for "as long as it takes".
Critics claimed this created the risk of indefinite detention without trial, but police claimed it may be necessary in complex cases.
Speaking to Radio 4's Today programme, Lord West said Islamist groups posed a "real threat to this nation".
He revealed the security services are monitoring 2,000 terror suspects and a further 2,000 sympathisers.
"The scale of this whole thing is quite dramatic", he said, adding around 30 gangs are actively preparing for attacks.
Lord West said: "I have come back to this after 16 months away from it and I have to say I was quite concerned when I saw what the level of threat is.
"If one looks at what our security service and police are looking at on a day-to-day basis, they are now monitoring over 200 groupings or networks which to varying degrees are threatening our security.
"There are 30 that are actually being looked at very closely indeed because they have got to the stage where they are gathering materials and doing things which could lead in the fairly short term to doing something if they wanted to."
He said the security services were becoming more adept at detecting threats but were then having to analyse increasingly complex information, requiring an extended detention period.
"I think, looking at the complexity of this, there will be occasions when we need more than 28 days," Lord West said. He admitted no case had yet taken more than 28 days but said one had come "right to the wire".
Former home secretary John Reid warned parliament needed to reconsider the limit and Gordon Brown has not ruled out raising it to 90 days.
Nick Clegg, Liberal Democrats home affairs spokesman said real evidence would be required before limits could be increased.
Mr Clegg said: "The police got themselves and the government in terrible trouble last time by arguing for 90 days without any real evidence to back their case.
"So it is bizarre in the extreme for ACPO to appear to be repeating the mistake by floating the need for indefinite detention without charge, again without any overwhelming evidence that it is necessary."
Tony Blair suffered his first Commons defeat over the previous attempt to impose a 90-day detention without charge limit. The 28-day limit was a compromise measure and doubled the previous maximum period of 14 days.
Shadow home secretary David Davis said yesterday the government had been wrong to push for a 90-day limit and said the Conservatives were absolutely opposed to Mr Jones' suggestion terror suspects should be held for "as long as it takes".
Mr Davis said: "Since the 28 day limit was introduced neither the police or security services have produced one shred of evidence to demonstrate the need for extension, either in public or in confidential briefings."
Lord Carlile said the political debate over detention time limits was "completely sterile". The Lib Dem peer, who reviews counter-terrorism measures, argued the decision should be handed to senior judges.