It is understood that the PM attended a leaving event for the outgoing Director of Communications Lee Cain on November 13 2020, according to sources.
While at the gathering, Mr Johnson allegedly made a speech and stayed for “some time”, although it is not known whether he consumed alcohol.
Given the amount of time he allegedly stayed for, it will be harder for him to argue it was a work event and he had a “reasonable excuse” for attending, reports state.
The Sue Gray report is likely to be critical of the PM’s attendance at the event, making it more likely he will once again be fined, a report has suggested.
It was announced this week that the Prime Minister and Chancellor Rishi Sunak would both receive fixed penalty notices for attending parties while the rest of the country was in lockdown during the COVID-19 pandemic.
Mr Johnson received a £50 fine for a birthday party he went to in June 2020 for less than 10 minutes, although a source close to the investigation said the leaving party for Mr Cain could have far more “serious” consequences.
The source told The Telegraph: “The event that took place on November 13, 2020, to mark the departure of Mr 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.”
Mr Johnson is accused of attending six out of the 12 events between May 2020 and April 2021 that Scotland Yard are investigating.
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“I hope Conservative MPs will finally do their patriotic duty and kick Boris Johnson out of Number 10.”
A source added: “We are actively engaged with the Speaker’s Office in relation to raising a complaint of breach of privilege or contempt, as per Erskine May [the Common’s rule book].”
If Mr Johnson is found guilty he could be suspended or even expelled from the House of Commons.