In a speech to the Policy Exchange thinktank Mr Clarke stressed that it was "realistic" to expect terrorists to strike Britain again in the aftermath of the July 7th 2005 London bombings.
"We suffered the appalling attacks of July 2005, and the only sensible assumption is that we shall be attacked again," the head of Scotland Yard's counter-terror operations said.
"This is a depressing prospect, but is no more than a realistic assessment of the complexity of the threat we face," he added.
Mr Clarke, who said that there were currently more than 100 people in the UK awaiting trial on terror charges, also warned that militants appeared to be planning "more ambitious" attacks, as demonstrated by the foiled plot to blow up transatlantic airliners, which was made public by police last summer.
International terror network al-Qaida had also proved "incredibly resilient" to global efforts to break it up, Mr Clarke said, remarking that the group had "certainly retained its ability to deliver centrally directed attacks" in the UK.
The warning comes ahead of a planned speech by home secretary John Reid today, in which he is expected to argue that splitting the Home Office will boost Britain's security and provide a more effective means of responding to the terror threat.
However Mr Clarke also warned last night that some anti-terror investigations were being compromised by intelligence leaks, describing those responsible for unofficially releasing sensitive information to the media as "beneath contempt".
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