Why has he gone?
By Dominic Casciani
BBC News home affairs reporter
Britain's top police officer, Sir Ian Blair, has faced more than three years of battles over his leadership of the Met.
Throughout all this time he has resisted any attempts to remove him from his post. In recent weeks he has faced a slew of new allegations, including an internal race row at Scotland Yard and claims of improper awarding of contracts.
But he says that none of this was behind his decision to quit. He says he has gone because he did not have the support of the mayor of London, Boris Johnson.
This week Mr Johnson took over chairmanship of the Metropolitan Police Authority - a power his predecessor Ken Livingstone did not use because it came in just before he left office.
In his statement, Sir Ian said that at their first meeting the new mayor made it "very clear" that the commissioner did not have his full support.
Sir Ian said that while he knew he had the support of the home secretary, Jacqui Smith, he could not do the job without the support of the mayor.
So has he immediately walked out?
No. He will be staying in post until 1 December to allow the Home Office, Mayor of London and Metropolitan Police Authority to begin the process of finding his successor. Sir Paul Stephenson, Sir Ian's deputy, will be acting commissioner, if required.
Can London's mayor remove the commissioner?
The mayor has no power over the post. Each commissioner is appointed by the home secretary because of the unique responsibilities that come with the post.
The commissioner combines the huge pressures of running the capital's police force with key national roles.
Sir Ian was the de facto head of the British police, the key man who politicians rely on to help shape crime policy.
The commissioner is responsible for counter-terrorism, with a national team of investigators working in New Scotland Yard.
So the mayor may disagree with the commissioner over policing the streets of London - but the commissioner is serving a much more powerful master.
If that's the case, why has he gone?
Sir Ian has concluded that politically he had no other choice. "Without the mayor's backing, I do not consider that I can continue in the job," he said.
"Personally I see no bar to working effectively with the new mayor, but it is there that we differ and hence I am unable to continue."
What allegations has Sir Ian faced?
Sir Ian has faced attack after attack during his three years in post. There were calls for him to go after the mistaken shooting of Jean Charles de Menezes at Stockwell Underground station.
He weathered that storm and renewed calls when the force was found guilty at the Old Bailey in relation to the killing.
In the last few months he was embroiled in a massive race row with two senior Asian officers, one of whom he suspended.
It also recently emerged that there was an ongoing investigation into how a company owned by one of Sir Ian's friends had won contracts at Scotland Yard.
Did he still have government's support?
Yes. Successive home secretaries have stood by Sir Ian, believing him to be a highly capable policeman and a crucial figure in Whitehall's attempts to modernise Britain's constabularies.
Ms Smith's letter to Sir Ian, accepting his resignation, is telling. "It is the job of responsible politicians to support those who have to carry heavy operational burdens. It has been my honour to do so."
Did Sir Ian have the support of the rank and file?
Sir Ian won friends in politics because he wanted to banish old-style policing attitudes - and some officers didn't like that. They thought his reforms were getting in the way of keeping the streets safe.
However, he introduced neighbourhood policing which was designed to help officers forge strong links with local people, leaving them better placed to crack crime and anti-social behaviour. Ministers say this strategy is behind London's dramatic drop in crime.
Ultimately, many officers accused Sir Ian of being a politician first and a policeman second. But when it came to winning a political fight for his life, the policeman came second.
What happens next?
The Met Police Authority and the Home Office draw up a shortlist for interviews. The successful candidate has to pass two interviews - one with an MPA panel and another with the home secretary.
The home secretary must listen to the mayor - but it is ultimately her choice.
The question is whether Boris Johnson's crucial role in Sir Ian's ejection means that policing and politics are now bound together in a way no commissioner has ever seen before.