Islamist terrorists pose the greatest threat to Britain's security since the Second World War, the Metropolitan Police Commissioner has warned.
With police and security services on heightened alert for a possible attack over Christmas and the New Year, Sir Ian Blair echoed fears in the intelligence community that the risk of an atrocity has increased substantially in recent months.
He said he had no specific intelligence about a plot to target Britain over the holiday period, but added: "The threat of another terrorist attempt is ever present. Christmas is a period when that might happen. We have no specific intelligence to do [with] that."
He added: "There was a terrorist plot in Germany against one of their Christmas markets in 2002, so it's a possibility."
Sir Ian also said the danger to the public was of an "unparalleled nature and growing".
"It is a far graver threat in terms of civilians than either the Cold War or the Second World War," he told BBC Radio 4's Today programme. "You have to go back to that threat - it's a much graver threat than that posed by Irish Republican terrorism."
The commissioner said the IRA usually did not want to cause mass casualties, did not want its attackers to die, gave warnings and was "heavily penetrated" by intelligence agents. "None of those four applies with al-Qa'ida and its affiliates," he said.
Sir Ian also acknowledged that efforts to infilitrate terrorist cells were at an early stage.
"It took 20 years to penetrate the IRA and I have no doubt the intelligence services will be attempting that now, but it is a more difficult and a much more recent phenomenon. Therefore some of the techniques we just do not have."
Last month, Dame Eliza Manningham-Buller, the MI5 director general, disclosed that the agency was tracking more than 1,600 members of terrorist cells and was monitoring 30 major plots.
John Reid, the Home Secretary, has also repeatedly warned that a terrorist attack in Britain is "highly likely". Sir Ian defended the bungled police raid in Forest Gate, east London, in June when a man was shot and injured by officers who were hunting for terrorist materials.
"We have apologised for the fact we did not find what we were looking for."
The commissioner also said the police had learnt their lesson from Forest Gate when they thwarted an alleged plot in August to blow up transatlantic airliners. He said: "[We made] it clear to the public the scale of the threat we had uncovered, [the threat] we thought we had uncovered, and why we had to move forward."Reuse content