A source close to the “partygate” investigation said the June 2020 birthday party event over which Mr Johnson was fined £50, and which he attended for less than 10 minutes, is far less serious than the leaving party for Mr Cain.

The source said: “The event that took place on Nov 13, 2020, to mark the departure of Lee Cain is considered to be the most serious breach of the coronavirus regulations among the events that the Prime Minister attended.

“Given that he has already received a fine for what was a relatively minor breach, it seems inevitable he will receive more fines as the police complete their investigation.”

Scotland Yard is investigating a total of 12 events between May 2020 and April last year, with Mr Johnson accused of being present at six of them.

The news came as it emerged that he is facing parliamentary investigation into whether he misled the Commons and that Sir Lindsay Hoyle, the Speaker, supports the move.

Westminster sources confirmed that Sir Lindsay is likely to grant a debate as soon as Wednesday next week, when MPs will vote on whether to refer Mr Johnson’s conduct to an investigation by the privileges committee.

The seven-MP committee – which has a Tory majority but is chaired by Chris Bryant, a Labour MP – will then rule on whether Mr Johnson has been found in contempt of Parliament.