Blair's announcement came a day after Johnson assumed the chairmanship of the Metropolitan Police Authority, The Telegraph reported. Blair said that Johnson, "in a pleasant but determined manner," told him a leadership change is needed.

"Without the mayor's backing I do not think I can continue in the job," Blair told reporters at a news conference.

Blair, a graduate of Oxford University, joined the Met, popularly known as Scotland Yard, in 1974 as a constable. He rose through the ranks to chief superintendent and then held a series of top administrative jobs before becoming commissioner in 2005.

Less than six months later, Blair became involved in one of the most controversial incidents of his career when Jean Charles de Menezes, a young Brazilian immigrant, was shot dead by mistake by officers on an anti-terrorist stakeout in a Tube station. In 2007, a jury found that police had violated health and safety laws but concluded that the shooting was an isolated incident.