Credit where its due, it does look as if Iain Dale was first with the rumours, but thus far it seems to be The Times that has the most detailed coverage…

Sir Ian Blair is expected to announce his resignation as Metropolitan Police Commissioner this afternoon.

Sources told The Times that the pressures of recent months had forced Sir Ian, who became commissioner in February 2005, to consider his position.

He is understood to be standing down before Boris Johnson, the new Tory Mayor of London, takes control of the Metropolitan Police Authority next week.

“It is a combination of things – a new mayor in London who does not support him, the inquest into the death of Jean Charles de Menezes and the inquiry into how Scotland Yard contracts were awarded,” said a senior police source.

It was disclosed earlier today that an inquiry into Scotland Yard contracts awarded to a close friend of Sir Ian had uncovered one deal which had not involved a tendering process.

UPDATE – 17:38

Metropolitan Police Commissioner Sir Ian Blair has announced his resignation after three years in the job.

Sir Ian said he had lost the backing of London mayor Boris Johnson, who took over as chairman of the police authority on Wednesday.

Mr Johnson praised his “distinguished” service but said the Met would benefit from “new leadership”.