Met Police insiders insisted on Thursday that “partygate” was too far down the track for its outcome to be influenced and that too many officers were already involved. But the source accepted that Dame Cressida’s resignation will put “more pressure” on the officers in day-to-day charge of the case over whether Mr Johnson, as well as his wife Carrie and senior aides, receive fixed penalty notices for breaking the Government’s own Covid lockdown rules.

“There are loads of people involved in the investigation,” said the source, “The idea that the Commissioner could then influence an investigation to the extent that it brings about a corrupt and improper outcome is just fanciful. It is just beyond belief. It is fanciful.”

Boris Johnson must have ‘no role’ in choosing successor

Sir Ed Davey, the Liberal Democrat leader, said: “Boris Johnson must have no role in choosing Cressida Dick’s successor to lead the Met. A man under criminal investigation by the Met should not be able to choose who’s in charge of it.”

Dame Cressida had only announced the launch of a criminal inquiry into parties and gatherings in Downing Street a little over a fortnight ago on January 25. For months she had resisted a clamour to do so but the delivery of Sue Gray’s report into alleged illegal activities forced the Commissioner’s hand. Here was evidence of possible law-breaking that could not be ignored.

But the Met’s handling of the announcement was catastrophic. The Gray report was suppressed as a consequence, Scotland Yard requesting that “minimal reference” be made to 12 possibly criminal events in anything published by the senior civil servant. The result was an outcry that the Gray report had been kicked into the long grass, buying the beleaguered Prime Minister valuable time.

But the Met inquiry had been gathering pace. On Wednesday, the force announced that Operation Hillman – the codename for the “partygate” inquiry – would be contacting more than 50 people to ask them to explain their involvement before issuing fines. Mr and Mrs Johnson are likely recipients of the letters. Anybody getting one had seven days to offer a “reasonable excuse” for suspected criminal behaviour. After that a fixed penalty notice would follow.

When Dame Cressida says she will stay on for a “short period”, it raises the question of whether she is hanging around for the completion of Operation Hillman. In doing so that would take the heat off any incoming Commissioner or Acting Commissioner.

The suggestion is that she has been sacked almost accidentally by Mr Khan. On Wednesday, the Mayor had said he was prepared to oust her in “days or weeks” over a series of scandals engulfing Britain’s biggest force. A little over 24 hours later, Dame Cressida was gone. The pair had been due to meet at City Hall at 4.30pm on Thursday but it never took place.