Police have prevented a terror attack in London on the scale of the Madrid bombings, according to a police chief.
Sir John said he still believed an attack on London was inevitable
Speaking to BBC London on Thursday, Met Police Commissioner Sir John Stevens said terrorism was a major issue for the UK capital.
He said a number of terror attacks had been thwarted and hundreds of people were going through the courts.
"The risk of an attack to London has not changed; an attack is still inevitable," he said.
"Thank God to date, and we have had to work extremely hard, we've thwarted attacks, " he added.
When asked if the force had stopped an attack on the scale of Madrid he said: "Yes, I can't discuss it because of court proceedings but yes we have stopped a Madrid."
The 11 March attacks on four commuter trains in Madrid which killed 200 people was Europe's worst terror attack since the 1988 Lockerbie bombing.
Sir John also said one of the major issues affecting London was knives.
"We've got a big problem with knives," he said. "We've driven down gun crime but I think there has been a move over to knives.
"If people are carrying a knife for the wrong reasons, i.e. to inflict personal damage, then I think they should receive a mandatory sentence of two if not three years."
Sir John is stepping down as commissioner on 31 January and deputy commissioner Sir Ian Blair will be the force's next chief.
When asked what his biggest regret was during his time as commissioner Sir John said Damilola Taylor.
"Having been a career detective and having dealt with murders it was just the circumstances of that case.
"Any murder is tragic and horrible but having seen the family and having got close to them that was a case that haunts me in a way.
"We are still hopeful and will keep ploughing on."
Four youths were cleared of Damilola's murder at the Old Bailey in April 2002.