Boris Johnson “must have known” he had broken Covid rules, a damning report into Partygate will find tomorrow.
The shamed former PM repeatedly denied to Parliament there had been lockdown-busting gatherings in Downing Street.
But the Privileges Committee will conclude that Mr Johnson had deliberately misled MPs over the parties.
It is expected to publish a 30,000 word report into the saga on Thursday morning.
Mr Johnson announced his resignation as an MP last week after being informed by the committee he faced being suspended from the Commons and a possible by-election in his Uxbridge and South Ruislip constituency.
Since he has already quit, it may choose to publicly shame him rather than hand down a punishment.
Or it could recommend withholding the Commons pass given to most former MPs allowing them continued access to the parliamentary estate.
Mr Johnson has launched a last minute attempt to discredit the committee by lashing out its members.
He latched onto claims that Tory MP Sir Bernard Jenkin had attended a drinks party in the House of Commons in December 2020, while London was in Tier 2 measures restricting indoor mixing.
In a statement, Mr Johnson said: "Bernard Jenkin has just voted to expel me from Parliament for allegedly trying to conceal from Parliament my knowledge of illicit events.
"Now it turns out he may have for the whole time known that he himself attended an event - and concealed this from the Privileges Committee and the whole House for the last year.
"To borrow the language of the committee, if this is the case, he 'must have known' he was in breach of the rules.
"He has no choice but to explain his actions to his own committee, for his colleagues to investigate and then to resign."
Liberal Democrats deputy leader Daisy Cooper said: "This a typical distraction tactic from Boris Johnson that doesn't change the fact he broke the law and lied about it."The Conservative Party is now in a full blown civil war, while people struggle to afford to pay their mortgage or get a GP appointment. This whole unedifying spectacle needs to brought to an end as soon as possible.
"We need a general election now to finally get rid of this chaotic Conservative government."
The latest round of Tory infighting came as Nadine Dorries attempted to prolong Rishi Sunak’s by-election misery by refusing to stand down immediately as an MP.
The former culture secretary announced on Friday she was resigning after she was stopped from getting a peerage.
Hours later Mr Johnson and pal Nigel Adams also announced they were quitting.
The Tories had hoped to hold all three by-elections to replace them on one day.
But Mrs Dorries is refusing to formally resign from her seat, meaning there will be a delay in holding a by-election in her Mid Bedfordshire seat.
She is angry with Mr Sunak who she has denounced as a “privileged posh boy” and accused of acting in a “duplicitous and cruel” way to stop her peerage.
Asked if there is frustration over the delay, Mr Sunak's press secretary said: "It's obviously unusual to have an MP say they will resign with immediate effect and for that not to take place.
“The Prime Minister believes the people of Mid Bedfordshire deserve proper representation in this house and he looks forward to campaigning for the Conservative candidate in the by-election."
By-elections are expected to take place in Mr Johnson's Uxbridge and South Ruislip constituency and Mr Adams' Selby and Ainsty seat on July 13 or 20.
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