The Prime Minister will make his first public appearance since the leak on Monday of an email from his principal private secretary Martin Reynolds inviting Downing Street staff to the gathering in May 2020.
The disclosure triggered a new wave of public anger following the reports last year of parties in the run up to Christmas 2020, with Tory MPs openly warning Mr Johnson his position will be untenable if he has been shown to have lied.
Downing Street has refused to say if he was present at the May event, despite reports he and his fiancee (now wife), Carrie Symonds, were among around 30 people to attend at a time when such gatherings were banned.
The Prime Minister has said it is a matter for Sue Gray, the senior civil servant who is investigating a series of reported parties in Downing Street and elsewhere in Whitehall in the course of 2020 to determine what happened.
However Conservative MPs warned that such a position was simply unsustainable as Mr Johnson must know whether he was there or not.
Backbencher Nigel Mills warned that any senior figure who willingly attended the event could not have a position where they were responsible for setting Covid-19 policy.
“It is utterly untenable, we have seen people resign for far less than that. If the Prime Minister knowingly attended a party, I can’t see how he can survive,” he told BBC News.
“I don’t think we need an inquiry to work out whether the Prime Minister was there. He knows whether he was there or not. Just come out and say what happened.
“If he was there he better try a hugely fulsome apology and see if the country will buy it but I’m not sure they will.”
His comments echoed the leader of the Scottish Tories Douglas Ross who again warned that Mr Johnson could not carry on in No 10 if he was found to have misled Parliament.
Sir Charles Walker the vice chairman of the Conservative backbench 1922 Committee, said there was a lot of anger over what had happened and said the Prime Minister urgently needed to rebuild public trust.
“I think the Prime Minister needs to spend the next six months restoring trust in No 10 and making some good and strong decisions. I think that is the challenge for him,” he told Channel 4 News.
There was frustration among Tories at the failure of Downing Street to address the latest allegations on Tuesday.
Mr Johnson avoided scrutiny by sending the Paymaster General Michael Ellis to respond on his behalf to an emergency Commons question on the issue tabled by Labour.
In a possible indication of waning support among Tory MPs for their leader, the Conservative benches were sparsely populated while no senior ministers appeared publicly to defend him.
In an echo of what happened last year when Health Secretary Sajid Javid pulled out of a morning broadcast round following earlier disclosures, there was no sign of a minister taking to the airwaves on Wednesday.
In contrast, the Commons chamber is expected to be packed for Prime Minister’s Questions as MPs watch to see if he can turn round an increasingly perilous position.
With the public mood turning increasingly angry, two snap polls found a majority now believed Mr Johnson should stand down as Prime Minister.
A Savanta ComRes study found 66% of British adults thought he should quit, with 24% saying he should stay while a YouGov survey for Sky News found 56% believed he should go, with 27% saying he should remain.
Meanwhile, Scotland Yard has said it is in contact with the Cabinet Office about the latest allegation.
As a result, Ms Gray’s investigation could be paused if evidence emerges of a criminal offence and the Metropolitan Police decide to launch an inquiry.