The commissioner of the Metropolitan Police has warned that a terror attack in the UK is a case of "when, not if".
Sir Bernard Hogan-Howe has said that he cannot promise an attack will not take place, despite being in charge of preventing them.
Writing in the Mail on Sunday he said instead that he could offer reassurance by explaining what the police and security services were doing to protect the public.
Speaking of the terrorist atrocities Europe has seen recently, 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."
Explaining the work of the police, he pointed to the number of terror plots that had been foiled since the murder of Lee Rigby in May 2013, including one to murder officers at Shepherd's Bush police station, west London, and another to carry out a Lee Rigby-style attack on US soldiers in East Anglia.
He said the relationship between the police, MI5 and MI6 was a "world-beater" which has given the UK an intelligence advantage and said the nation's gun control laws helped make it difficult for terrorists.
The UK's top officer also said that the "British way of life and culture" made the UK hostile to terrorists.
He said the fact most British police are unarmed gave the public a healthier relationship with officers, helping neighbourhood officers become the nation's eyes and ears, and praised the UK's tolerance and acceptance.