Sir Ian, widely considered the most politically correct Chief Constable in the history of the Met, admits in his resignation statement that he no longer enjoys the confidence of the Mayor and, with lukewarm support from Home Secretary Jacqui Smith, he felt it was in the best interests of the people of London that he should go.

Policing such an ethnically and culturally diverse city as London was never an easy task for Sir Ian Blair, who was appointed by Home Secretary David Blunkett in February 2005, and some of the difficult problems that he faced in staffing such an expansive force; in tackling Islamic-based terrorism and in dealing with racial tensions within the ranks combined to make his position untenable.

The controversy over the botched police shooting of innocent Brazilian man Jean Charles de Menzes landed firmly on Sir Ian's desk and he was also pilloried, unfairly perhaps, when the Met's Assistant Commissioner Tarique Ghaffur decided to sue him for racial discrimination.

Northern Ireland interest in Sir Ian's demise as Metropolitan Chief Constable heightened last night with speculation that the present PSNI Chief Constable Sir Hugh Orde was emerging as a possible successor, and former RUC Chief Constable Sir Ronnie Flanagan was also linked with the job.

After seven years in Northern Ireland, Sir Hugh Orde, who once had a senior detective role in the Met, may feel that now is the time to move on to a higher position.

Sir Hugh has still two years of his PSNI contract to complete, but with his considerable expertise and detailed knowledge of how policing works in the various boroughs of London, he would certainly have a head start over most of his contemporaries applying for the post.