Sir John has been Metropolitan Police Commissioner since 2000

He said he was immensely sad to leave his job after a 43-year career in the police, and five as Met chief.

Sir John, 62, attended a passing out parade of the capital's latest police recruits in Hendon.

He also opened the John Stevens Centre communications building in Hendon, named in his honour.

Sir John said: "This is just unbelievable. I came to this training centre in October 1962 as a raw recruit.

"To think I am here today as commissioner with this centre named after me is beyond belief. It is a massive honour."

The centre is part of three buildings built to help the police respond to emergency calls.

He said: "Nearly 43 years of my life have been wrapped up in police work. It is very sad, very emotional."

Sir John told BBC London he was proud the Met had increased its numbers under his command.

Terrorism threat

"When I became commissioner, we had 25,400 officers," he said.

"Scandalous - there had been a reduction in policing numbers in this city which was totally unacceptable."

He also said it was wrong there was confusion over the laws dictating what kind of force homeowners could reasonably use against burglars.

Sir John caused some controversy when he said the capital was vulnerable to attack by international terrorists.

"I'll say it as it is," he said. "Yes, London has been under attack.

"Fortunately, the massive amount of hard work that the security services and we and the police have done, have thwarted those attacks."

Sir John said the Met needed to be tougher at rooting out corrupt officers - his predecessor Sir Paul Condon said there were more than 200 in the force.

He said there needed to be a system that "identifies, prosecutes and convicts them".