LONDON, Oct 3, 2008 (AFP) - Britain's top policeman announced his resignation Thursday after long-running criticism over the shooting of an innocent man, allegations of racism and open discord with London's new mayor.

Metropolitan Police chief Ian Blair, who took office shortly before the devastating July 2005 suicide bombings in London, said he had lost the backing of the capital's mayor Boris Johnson, who was elected in May.

'I have today offered my resignation as commissioner to the home secretary which she has reluctantly but graciously accepted,' he told a hastily arranged press conference, adding that he would stand down on December 1.

He said Johnson had made clear he wanted a change, and 'without the mayor's backing I do not consider that I can continue in the job'.

Prime Minister Gordon Brown paid tribute to Blair, saying he had made a 'huge personal contribution to the safety and security of our country' and noting in particular his leadership during the London attacks.

Home Secretary Jacqui Smith also expressed regret at Blair's departure and criticised Johnson's behaviour, telling the BBC there was 'a process in place' to deal with any concerns 'that the mayor chose not to respect'.

Former mayor Ken Livingstone, a member of the ruling Labour party, said it was a 'political decision'.

But Johnson, an opposition Conservative, said the Metropolitan Police would benefit from a change of leadership, 'a clean break and a new start'.

'I have made it clear that there comes a time in any organisation to give new leadership,' he said. There had been 'no particular story or particular allegation that was uppermost in our consideration,' he added.

Blair has long faced criticism over the death of Jean Charles de Menezes, an innocent Brazilian shot dead by police who mistook him for a suicide bomber in the wake of July 2005 bombings in London, which killed 56 people.

Other issues have added to the pressure -- in August the Met's top Muslim officer, Assistant Commissioner Tarique Ghaffur, said he was taking Blair to an employment tribunal over alleged racism.

And a report in the Daily Mail newspaper Thursday claimed Blair used public money to pay a close friend more than 15,000 pounds (26,400 dollars, 19,000 euros) to help him sharpen up his image when he became Met chief in 2005.

The De Menezes killing has been the main cloud hanging over him. Scotland Yard was heavily criticised in a report in August 2007 on the killing, although Blair himself escaped censure.

The pressure increased last November after his police force was found guilty of breaching health and safety laws in the shooting, but he fought off calls to quit and said officers had done their best a 'simply extraordinary situation'.

De Menezes' family expressed shock Thursday at Blair's resignation halfway through the three-month inquest into the Brazilian's death.

In a statement, they said Blair 'should have been ultimately accountable' for the killing, but his resignation did not acknowledge any failings.

'We believe he certainly bears responsibility for the lies told about Jean and the cover-up by police in the aftermath of the shooting,' the family said.

The populist Daily Mail celebrated Blair's departure, condemning 'his extraordinary combination of obsessive political correctness and stunning ineptitude has led to a creeping paralysis at the top of Britain's most important police force'.

But the left-leaning Guardian denounced a 'Tory plot', saying that while Blair's days were numbered because of the De Menezes shooting, his departure raised 'serious questions' about the politicisation of the police force.

Blair's deputy Paul Stephenson, 54, will take over as acting commissioner, while favourites for the job include Hugh Orde, who heads Northern Ireland's police service, and Ronnie Flanagan, the government's top policing advisor.