Metropolitan Police Commissioner Sir Ian Blair has announced his resignation, blaming a lack of support from London mayor Boris Johnson.
Sir Ian said that “without the mayor’s backing I do not think I can continue”.
Mr Johnson, who took over as chairman of the police authority on Wednesday, praised his service but said the Met would benefit from “new leadership”.
Sir Ian, who became the UK’s top police officer in February 2005, said he would be stepping down on 1 December.
Prime Minister Gordon Brown said Sir Ian had made a “huge personal contribution to the safety and security of our country”.
He paid tribute to Sir Ian’s leadership at the time of the July 2005 suicide bomb attacks on London’s transport system.
Home Secretary Jacqui Smith said deputy commissioner Sir Paul Stephenson would take over as acting head of the Met should no successor be found by 1 December.
Ms Smith said: “I pay tribute to Sir Ian for the massive reductions in crime that his leadership of the Met has overseen and his continuing efforts to tackle gun, gang and knife crime.”
“His part in leading neighbourhood policing across London has led to Londoners being safer and more confident.”
Mr Johnson, who has repeatedly avoided publicly backing Sir Ian since being elected mayor in May, had called for the commissioner to be directly accountable to City Hall.
Speaking after Sir Ian’s resignation, the mayor said: “There comes a time in any organisation when it becomes clear it would benefit from new leadership and clarity of purpose. I believe that time is now.”
‘No secrets’
Sir Ian, who became the UK’s top police officer in February 2005, said he would be stepping down on 1 December.
Defending his record, Sir Ian said: “I am resigning not because of any failures of my service and not because the pressures of the office and the many stories that surround it are too much.
“I am resigning in the best interests of the people of London and of the Metropolitan Police Service.”
He said he had wanted to stay on until his contract ran out in February 2010.
“However, 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.”
Sir Ian’s tenure has been dogged by controversy.
MPA auditors are in the process of examining Scotland Yard contracts given to consultancy firm Impact Plus, run by a friend.
Sir Ian has said he had been “open and straightforward in informing both the MPS [Metropolitan Police Service] and the MPA about my relationship with someone who was subsequently awarded a contract with the MPS”.
Sir Ian has also faced criticism over the racism row involving the Met’s most senior Asian officer Tarique Ghaffur.
There have been questions too about his handling of events surrounding the 2005 death of Jean Charles de Menezes, who was shot dead at Stockwell Underground station in south London after being mistaken for a suicide bomber.
The Met Police were later convicted of a health and safety offence over the incident.
Erionaldo da Silva, speaking on behalf of the de Menezes family, said Sir Ian should have resigned three years ago and the decision to do so now should not deflect attention from Jean Charles’ ongoing inquest.
Shadow home secretary Dominic Grieve said Sir Ian had taken the “right decision” in standing down.
“We have been calling for Sir Ian to step down for almost a year – since the serial and systematic failings at the Metropolitan Police [service] disclosed during the de Menezes trial – whilst cabinet ministers from the PM onwards continued to express total confidence in him.
“It is now clear that they have shown a serious lack of judgment about the leadership of the most important police force in Britain.
“It is vital that a successor is appointed who can restore public confidence.”
Lib Dem home affairs spokesman Chris Huhne said Sir Ian “had become part of the Met’s problem, not its solution”.
“His resignation is long overdue following a string of embarrassments for his force… the Met now needs a tough professional, not a wannabe politician.”
The former London mayor Ken Livingstone said the circumstances of the resignation appeared to be a political decision.
“Whoever now takes the job as Sir Ian’s successor will know that they may be asked to leave at a change of election.”
Sir Edward Henry was the last commissioner of the Metropolitan Police to quit in 1918. His departure came after the hugely damaging police strike of that year, the last time officers were allowed to walk out.
Sir Ian Blair Video:
Sir Ian Blair says leading the Met Police was ‘the proudest time’ of his life
BBC 2 October 2008