The Metropolitan Police chief has warned that a terror attack in the UK is a case of "when, not if".
Sir Bernard Hogan-Howe, commissioner of the Met, said that he cannot promise an attack will not take place, although he is in charge of preventing them.
He added that he could offer some reassurance by telling the public how police and security services were working to protect them.
Writing about recent atrocities elsewhere in Europe, Sir Bernard said: "I feel and understand that fear, and as the police officer in charge of preventing such an attack I know you want me to reassure you.
"I am afraid I cannot do that entirely. Our threat level has been at 'Severe' for two years. It remains there.
"It means an attack is highly likely - you could say it is a case of when, not if."
He said that a number of terror plots had been foiled since the murder of Lee Rigby in May 2013, including one to carry out a similar attack on US soldiers in East Anglia.
The relationship between the police, MI5 and MI6 was among the best in the world, giving the UK an intelligence advantage, he added.