Boris Johnson is facing the key moment in his fight against suspension from Parliament over Partygate as Rishi Sunak and Dominic Cummings are dragged into the row over whether the ex-prime minister lied to MPs.

In a four-hour evidence session on Wednesday, Mr Johnson will be questioned over why he assured the Commons that No 10 staff had followed Covid rules and guidance when they gathered for drinks in Downing Street.

In a 52-page dossier published on Tuesday, he insisted that the case against him was founded on the arguments of the “discredited” Mr Cummings, who may now be called to give evidence to the committee.

Mr Johnson also cited Mr Sunak in his defence, arguing that the two men should not have been fined for attending a brief birthday party in the Cabinet room. The former PM wrote: “It remains unclear to me – and I believe the Prime Minister may feel the same – how precisely we committed an offence under the regulations.”

The current Prime Minister has sought to distance himself from the investigation but i has been told he was unsurprised to see himself mentioned in the dossier. A spokesman repeated Mr Sunak’s previous apology for breaking the rules.

Ahead of Wednesday’s hearing, Mr Johnson said: “I believe that the evidence conclusively shows that I did not knowingly or recklessly mislead Parliament. The committee has produced not a shred of evidence to show that I have.”

A source said he was “very confident” about the outcome of the inquiry, which could see him suspended from the Commons and potentially facing a by-election in his Uxbridge and South Ruislip constituency if he is found to have lied.

Those who worked with Mr Johnson in No 10 believe one key piece of evidence will be the fact that he commissioned a Civil Service investigation – eventually led by Sue Gray – into the Partygate allegations shortly after he emerged, suggesting he was confident he would be cleared of wrongdoing.

The Privileges Committee, made up of a cross-party group of MPs, has not ruled out calling further witnesses to give oral evidence after it questions the former PM – although the move is seen as relatively unlikely unless anything new appears in the hearing.

The witnesses could include Mr Cummings, former communications chief Jack Doyle, and other key figures named in Mr Johnson’s written evidence.

A fresh cache of documents is due to be published at 9am on Wednesday which could throw further light on the case. Insiders said it consisted of evidence that the committee and Mr Johnson will refer to in the course of the hearing – including witness statements, emails and WhatsApps from the heart of Downing Street during the Partygate period and its aftermath.

Supporters of the former Tory leader claimed his dossier had shown he was not guilty of lying but had instead inadvertently misled Parliament because he was wrongly briefed by his staff.

An ex-Cabinet minister said: “It’s very compelling evidence and clear that Boris did not knowingly mislead Parliament. I suspect any objectively minded committee would reach the same conclusion, but we’ll see.”

But other Conservative MPs are reserving judgement until the investigation has concluded. One said: “The problem is, there clearly were parties. He’s admitted he misled Parliament. On all of them, did he really not know that what he was saying was wrong? I think it’s going to be very difficult for them to just close their eyes to that.”

A backbencher added: “He’s lucky he isn’t being questioned by a real lawyer, because they’d get forensic. MPs ask political questions.”

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