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.
The so-called “vanity contract”, to promote the commissioner’s image when he took over from Lord Stevens of Kirkwhelpington, was worth £15,000 to Impact Plus, a consultancy run by Andy Miller, a longtime friend of Sir Ian.
The award of £3 million of contracts to Impact Plus is under investigation by a team of officers led by Sir Ronnie Flanagan, HM Chief Inspector of Constabulary.
Sir Ian, 55, has also been accused of racial discrimination by Assistant Commissioner Tarique Ghaffur, his most senior Asian officer.
Mr Ghaffur has been ordered to take leave pending an employment tribunal.
If Sir Ian decides to resign his departure will destabilise the Met at a time when it is facing searching questions at the inquest into the death of Mr de Menezes.
He was shot dead by armed officers who mistook him for a suicide bomber in July 2005, and a parade of senior officers is currently being called as witnesses at the inquest into the death.
Sir Ian is said to feel dispirited at the lack of backing he has received from Mr Johnson as the allegations against him have mounted.
The commissioner has robustly defended himself against allegations of wrongdoing over the award of contracts to Mr Miller but is understood to have felt increasingly isolated.
Mr Johnson, who called for Sir Ian to resign before his election as mayor, chairs his first MPA meeting next Monday.