BORIS Johnson is set to face perhaps the biggest challenge yet to his reign over allegations of law-breaking boozy parties at No 10.

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 to “make the most of the lovely weather”.

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 Johnson his position will be untenable if he has been shown to have lied.

The Tory leader 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.

READ MORE: Downing Street staff told to 'clean up' phones amid claims of parties at No 10

However, Conservative MPs warned that such a position was simply unsustainable as Johnson must know whether he was at the “socially distanced drinks” on May 20 2020 or not.

No ministers were on the airwaves on Wednesday morning to answer questions about the partygate row in a sign of the nervousness in Downing Street about the situation.

But 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.

Huw Merriman, chairman of the Commons Transport Committee, told BBC Radio 4’s Today programme that “more clarity is needed because we’re back where we were a month ago before the inquiry was set up where people are demanding answers”.

“We’re all in the dark – and that includes me,” he said.

But he suggested the Prime Minister would not necessarily need to resign over the issue, because “as far as I’m concerned, we judge people in the round”, looking at the whole of his record.

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 the BBC.

“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.”

The National: Douglas Ross

His comments echoed the leader of the Scottish Tories, Douglas Ross, who called for the Prime Minister to come clean about whether he attended the event and again warned that Johnson could not carry on in No 10 if he was found to have misled Parliament.

Backbencher Neil Hudson said he was “appalled and shocked” by the reports, adding “if rules have been broken then quite rightly there should be serious consequences”.

Senior Conservative MP Tobias Ellwood said the PM should “show some contrition”.

The Commons Defence Committee chairman told Sky News: “I strongly urge the Prime Minister to act now, to apologise for No 10’s poor judgment, to show some contrition and to be committed to appropriately respond to Sue Gray’s findings when they come out.

“We can’t allow things to drift, that is not an option.”

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Labour leader Sir Keir Starmer, the former director of public prosecutions, will face Johnson at PMQs after negative Covid-19 tests released him from self-isolation.

In an indication of the opposition’s line of attack, deputy Labour leader Angela Rayner told Today: “People have been reflecting about what was happening to them at the time in May and many people are still grieving their loved ones who they weren’t able to say goodbye to at the time, and to think the Prime Minister was laughing and partying is just unforgivable.”

Rayner, asked whether she thought the garden of No 10 constituted a workplace – meaning the Prime Minister and his staff might have a defence for being there – added: “Many key workers are NHS staff who were working very heavy shifts, 12-hour shifts with full PPE on – they didn’t break out into the garden with cheese and wine and ‘bring your own booze’ scenarios.

“They were working incredibly hard, watching people’s loved ones die, holding smartphones and iPads in front of them so they could say goodbye to their loved ones – it is not acceptable to say: ‘This is a workplace garden, so we all cracked open the bubbly because it was a really nice day’.

“Many people at the time understood the rules, and the rules were very clear.”

Ian Blackford, meanwhile, has called on Conservative MPs to oust the PM if he refuses to resign.

With the public mood turning increasingly angry, two snap polls found a majority now believed 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, Gray’s investigation could be paused if evidence emerges of a criminal offence and the Metropolitan Police decide to launch an inquiry.