Britain's top police officer reluctantly resigned yesterday following months of criticism of his leadership and his handling of major investigations, including terrorism cases.

Ian Blair, commissioner of the London Metropolitan police, said he was stepping down as head of the 31,000-strong force 16 months before his contract was due to expire after being asked to go by the city's new mayor Boris Johnson.

"I have today offered my resignation as commissioner to the Home Secretary," Mr Blair told a news conference. "I am resigning in the best interests of the people of London and the Metropolitan police service."

The decision follows months of negative headlines surrounding the 55-year-old, particularly the mistaken killing by undercover police of a Brazilian man, Jean Charles de Menezes, on the London Underground (subway) in July 2005.

Mr De Menezes was confused with a terrorism suspect and shot seven times in the back of the head a day after failed suicide bombings on the transport network and when the capital was still jittery after deadly suicide attacks two weeks earlier.

Mr Blair said he did not want to resign, but felt he couldn't go on without the support of the mayor.

"At a meeting yesterday, the new mayor made clear in a very pleasant and determined way that he wished there to be a change of leadership at the Met," he said. "Without the mayor's backing I do not consider that I can continue in the job."

Oxford-educated Mr Blair has had a stormy time in office since taking over the top police job in February 2005.

As well as the shooting of Mr de Menezes, he was accused of racial discrimination by Britain's highest-ranking Muslim police officer. He was frequently outspoken in defending his policies.

Mr Blair always rejected the criticism of his leadership and repeatedly stated he had no intention of resigning.