Sir Ian Blair, the Metropolitan Police commissioner, today warned that al-Qa'eda poses a greater threat to civilian life than the Nazis did during the Second World War.
Warning: Sir Ian Blair
Warning of a possible attack this Christmas, he said that the terrorist threat to Britain from Islamic extremists was "unparalled and growing".
He added that neither the IRA nor the Russians during the Cold War were willing to take civilian life to the same degree as al-Qa'eda.
But he appeared to backtrack slightly from comments made earlier this month by John Reid, the Home Secretary, that a terrorist attack over the festive period was "highly likely".
Sir Ian told the BBC Radio 4 Today programme that the police had "no specific intelligence", although the threat was "ever present".
He apologised for the raid in Forest Gate, east London, in which an innocent man was shot in the shoulder, and said he expected to be cleared by the Independent Police Complaints Commission (IPCC) investigation into the shooting of Brazilian Jean Charles de Menezes.
And he criticised the IPCC for allowing the investigation to drag on.
"It is very difficult to understand how an organisation can take 13 months to investigate what I either did or didn't say on one particular day," he said.
"I don't in any way impugn their integrity or their commitment but the system must be wrong to take that length of time."
Speaking of the IRA's tactics at the height the Troubles, he said that they did not want to cause mass atrocities, did not want to die, gave warnings and were "heavily penetrated" by British intelligence agents.
"None of those four apply with al-Qa'eda and its affiliates," he said.