Mounting pressure that meant Sir Ian had to go

Some of the most damaging allegations have erupted in recent weeks with a Metropolitan Police Authority investigation launched into Sir Ian's role in awarding contracts to one of his friends and accusations by Assistant Commissioner Tarique Ghaffur of racial, religious and age discrimination.

The Met Commissioner is also facing renewed pressure over his handling of the shooting of Brazilian Jean Charles de Menezes at Stockwell Tube station in July 2005 with the start of the inquest into the killing.

It is expected to raise fresh concerns about Sir Ian's management of the operation and its aftermath which has already led to the Met being convicted under health and safety laws.

Sir Ian's time as Britain's most senior policeman has also been blighted by a series of verbal gaffes. They include comments claiming that "nobody understood" why the Soham murders of schoolgirls Jessica Wells and Holly Chapman became a big story and a suggestion that falling crime in Haringey meant that residents now felt able to leave their doors unlocked.

Further pressure followed the Met Commissioner's admission that he had secretly tape recorded a number of telephone calls, including one with Attorney General Lord Goldsmith, while he was also accused of dragging the police into politics with his endorsement of the Government's 2005 bid to raise the pre-charge detention limit for terror suspects to 90 days.

Few would have predicted that so many allegations would hit Sir Ian, 55, when he took over at the Met after an impressive earlier career which had seen him hold senior posts in three forces.

Educated in Shropshire and Los Angeles, Sir Ian joined the Met as a graduate trainee in 1974 after finishing his degree at Oxford and started as a beat constable in Soho before rising rapidly through the ranks to become a detective chief inspector by 1985.

He subsequently led a major investigation into corruption within the force but then left to work for HM Inspectorate of Constabulary before taking up senior posts with Thames Valley and Surrey Police, where he served as the force's Chief Constable, and finally returning to the Met in 2000 as Deputy Commissioner.

He became Commissioner with apparently rosy prospects, having been knighted two years earlier for services to the police, and dubbed the "thinking man's policeman".

Among the successes that his supporters will point to are steady falls in crime in London during his tenure, with the most recent figures showing a five per cent drop in the number of offences in the capital during the last 12 months.

Ethnic minority recruitment has also risen sharply during Sir Ian's time as Commissioner, despite a recent spate of race discrimination cases launched by senior Asians within the force, while another plus has been his introduction of neighbourhood policing which has significantly increased the police presence on the streets.

Despite such positive developments, Sir Ian has rarely been popular either within the Met or outside the force and has regularly attracted criticism from all sides of the political spectrum.

Some have seen him as a "politically correct" government stooge, while on the Left his handling of incidents such as the Forest Gate arrests, when one man was accidentally shot and hundreds of police deployed during an unsuccessful anti-terror raid, has raised doubts about his ability to deal effectively with the issues of Islamic extremism and counterterrorism.

Last November's conviction of the Met over the de Menezes killing led to fresh calls for Sir Ian to quit and the publication soon after of a damning Independent Police Complaints Commission report into the shooting dented his support even further.

The head of the watchdog body later went on to say that Sir Ian's conduct had been responsible for much of the "avoidable difficulty" which had hit the force in the wake of the shooting.

Although Met officials tried to keep him out of the limelight, aware of his propensity to stir controversy with rash comments, he managed to anger the Director of Public Prosecutions Sir Ken Macdonald - and embarrass senior officers within his own force - with inaccurate comments criticising the failure to charge supermodel Kate Moss over pictures showing her apparently taking cocaine.

This succession of incidents, combined with frosty personal relations with some of his most senior colleagues, left support for the Met Commissioner ebbing away.

The arrival at City Hall of Boris Johnson, an avowed critic, coupled with the spate of recent allegations over contracts and racial discrimination today finally made his continuation in office untenable.