The UK’s most senior police officer is to stand down.

Sir Ian Blair has been Commissioner of the Metropolitan police for three years.

In a statement today he said that the Mayor of London, Boris Johnson, who took up the chairmanship of the Metropolitan Police Authority yesterday, had made it clear he wanted him to go.

In 2005 he clashed with Brian Paddick, his Deputy Assistant Commissioner, over when he was informed that armed officers had shot an innocent Brazilian man, and not a terrorist, at a Tube station two weeks after the attacks on London on July 7th.

Mr Paddick was the country’s most senior out gay officer until he retired in 2007.

He was the Liberal Democrat candidate for Mayor of London but lost to Mr Johnson in May.

Sir Ian said today:

“It is the duty of the Commissioner to lead the Met through good times and bad: to accept the burdens and pressures of office and, above all, to be a steward of the Service he commands.

“However, I have today offered my resignation as Commissioner to the Home Secretary, which she has reluctantly but graciously accepted.

“I am resigning not because of any failures by 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 the Metropolitan Police Service.

“I would have wished to continue to serve Londoners until my term of office expired in February 2010.

“However, at a meeting yesterday, the new Mayor made clear, in a very pleasant but determined way, that he wished there to be a change of leadership at the Met.

“I understand that to serve effectively the Commissioner must have the confidence of both the Mayor and the Home Secretary. Without the Mayor’s backing, I do not consider that I can continue in the job.

“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.”

Sir Ian will leave his post on December 1st when deputy commissioner Sir Paul Stephenson will take charge as the process of appointing a new Commissioner begins.

Assistant Commissioner John Yates, who led the investigation into cash for honours, is an early favourite for the job.

Today Boris Johnson praised Sir Ian’s 34 years of service.

“In the face of a chronic and dangerous terrorist threat, he can be very proud of his record in helping to keep millions of Londoners safe from harm,” the Mayor said.

“But 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.

“He leaves having made London safer and with our gratitude and best wishes. “

In December the independent police watchdog said that no officers from the Metropolitan police would face charges or disciplinary action over the shooting of Jean Charles de Menezes on July 22nd 2005.

Sir Ian insists that he did not know the 27 year old Brazilian was not a suicide bomber until the next day.

Mr Paddick said in evidence to an independent investigation that suspicions were raised almost immediately.

Today Mr Paddick told the BBC:

“There is a lot to be dealt with, a lot to be cleared out, if you will, and I think it needs a new broom to deal with those issues, rather than anybody who might be seen to be tainted by what has gone on in the last few months.

“I don’t think Londoners should be too concerned about a change at the top. I have said before, and I would maintain, that London will be better off with a new commissioner.

“I think some rank and file officers – those who felt that Ian Blair was a reformer, someone who was very pro-diversity – will be very disappointed to see him go.

“I think a lot of officers will see his position as untenable and will quite welcome the fact that there will be a new master at the helm.”

Shadow Home Secretary Dominic Grieve said it was right that Sir Ian leave the Met.

“We have been calling for Sir Ian to step down for almost a year – since the serial and systematic failings at the Metropolitan Police disclosed during the de Menezes trial – whilst Cabinet ministers from the PM onwards continued to express total confidence in him,” he said.

“It is now clear that they have shown a serious lack of judgement about the leadership of the most important police force in Britain.

“It is vital that a successor is appointed who can restore public confidence.”

Chris Huhne, the Liberal Democrat Home Affairs spokesman, also said it was time for Sir Ian to go,  “following a string of embarrassments for his force, including contracts given to friends, the shooting of Jean Charles de Menezes, accusations of racism causing chaos at senior levels and the politicisation of his role during the debates on detention for terrorist suspects.”

Home Secretary Jacqui Smith said that Sir Ian “can be proud of his contribution locally and nationally.”

“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,” she said.

In recent months the Commissioner has come under renewed pressure over the Met’s most senior Asian officer Tarique Ghaffur, who claims he has been the victim of racism.

Just last month he denied newspaper reports that his contract would not be renewed in 2010 and said he did not expect to have talks about continuing in the role until the middle of next year.

In November 2007 the Met were found guilty of breaching health and safety laws when they gunned down Mr de Menezes in Stockwell undergound station.

The Met was fined £175,000 and ordered to pay £385,000 costs.

The Prime Minister today said Sir Ian had made a “huge personal contribution” to the safety and security of the UK.

“I congratulate him and his officers, and of course I want to pay particular tribute to Ian’s leadership when London experienced the most serious terrorist attacks ever on British soil.”