Commissioner Bernard Hogan-Howe says his 31,000-member force has boosted its anti-terror resources since militants killed 130 people in the Nov. 13 assaults on Paris.
He says 2,800 London officers today carry guns, a quarter more than last year, while an elite counter-terrorist unit is on 24-hour standby.
All firearms-equipped officers, he says, have been trained "to take the initiative" to stop a Paris-style attack.
Hogan-Howe wrote in the Mail today newspaper that a London attack was "a case of when, not if." He wrote: "I know you want me to reassure you. I am afraid I cannot do that entirely.