Boris Johnson has been urged to recuse himself from picking Cressida Dick’s replacement as Met chief while he is under investigation over Partygate.
Lord Blair, a former Metropolitan Police Commissioner, described the decision about Dame Cressida Dick's successor as "an enormously important choice".
Dame Cressida dramatically announced she was standing down as Commissioner on Thursday evening after London Mayor Sadiq Khan made clear he had no confidence in her plans to reform the service.
Lord Blair told BBC Radio 4's Today programme: "That's a very difficult issue and I do accept that. I think Boris Johnson should recuse himself from being involved.
"But it's an enormously important choice and presumably it will take some time to get to being to who the commissioner is, and by that stage presumably he will have filled in his questionnaire and the matter will be over."
Downing Street confirmed on Friday night that Mr Johnson had received a legal questionnaire from Met officers investigating events in No 10.
He now has seven days to adequately explain his attendance or face a fine for breaking his own Covid regulations.
Meanwhile, Home Secretary Priti Patel has warned that the new Met chief must be prepared to tackle the "policing culture" which has left the country's biggest force reeling from a series of scandals.
Ms Patel, who must now oversee the appointment of a new Commissioner, said it would require "strong and decisive leadership" to rebuild public confidence in the force's "integrity and professionalism".
Dame Cressida's departure follows a barrage of criticism of the force including over its handling of the case of Sarah Everard who was murdered by a serving Met officer.
The force has also been criticised for being slow to investigate reports of parties in Downing Street and Whitehall in breach of Covid restrictions.
The final straw, however, was a report by the police watchdog which exposed violently racist, misogynist and homophobic messages exchanged by officers based at Charing Cross police station.
There were nevertheless signs of tension between Ms Patel and Mr Khan over the manner of Dame Cressida's departure, just months after the Home Secretary agreed a two-year extension to her contract.
Home Office sources said Ms Patel was angered by Mr Khan's failure to inform her that he had called Dame Cressida to a meeting on Thursday afternoon, which she considered "rude and unprofessional".
Dame Cressida, however, chose not attend after reportedly being informed that Mr Khan had no confidence in her plans for reform.
Sources close to the mayor said that it had been a regular bilateral meeting and that it was up to Dame Cressida to inform Ms Patel of her decision herself.
