Boris Johnson’s position will be “untenable” if he was found to have attended a drinks party in the No 10 garden in breach of Covid lockdown rules, a Conservative MP has said.

       The prime minister is under increasing pressure over the “bring your own booze” gathering – with backbenchers demanding he reveals whether he attended at PMQs on Wednesday.

       Nigel Mills MP said it would be resigning matter if he was found to have been at the 20 May 2020 event. “I would say anybody who organised or willingly attended a party at that time can’t possibly stay in position,” the backbencher told Times Radio.

       He added: “I can’t see how anybody who broke the rules in such a blatant way at the worst possible part of the pandemic possibly be remaining in a position to have any influence over Covid policy.”

       Mr Mills told BBC’s Newsnight the PM had “accepted resignations for far less,” adding: “He accepted the resignation of his spokesperson [Allegra Stratton] for not attending party but joking about it at a time of much lighter restrictions. I just think that’s untenable.”

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       Tory peer Ros Altmann also told Newsnight: “If those who were attending, and breaking the law, include the prime minister, then obviously the position becomes untenable.”

       Douglas Ross, leader of the Scottish Conservatives, has also warned that Mr Johnson would have to resign if he was found to have broken the rules. He called on the prime minister to tell the public whether he attended, rather than refer to Sue Gray’s ongoing investigation.

       Mr Johnson faces an uncomfortable grilling over party-gate by Labour leader Sir Keir Starmer at PMQs, who comes out of self-isolation on Wednesday after testing negative for a second consecutive day.

       Peter Cardwell, former special adviser to Mr Johnson, said the PM must “level with the country” today and explain whether he attended the drinks events.

       “Keir Starmer only needs to ask the same question six times – ‘Did you attend the party on 20 May 2020, yes or no?’ – and Boris Johnson must give an answer,” he told Sky News on today’s PMQs.

       Two sources told The Times that Mr Johnson attended the 20 May 2020 party and was seen “wandering round gladhanding people”. The newspaper claims one official even joked about the risk of a drone capturing the garden party from above.

       It comes as The Independent was told that No 10 staff were advised to “clean up” their phones by removing information that could suggest parties were held at No 10. Two sources claimed a senior member of staff told them it would be a “good idea” to remove messages.

       Asked about the story, Mr Cardwell said: “If anyone’s deleting any information that is very unethical and possibly criminal, and they absolutely should not be doing.”

       Another senior Tory backbencher told The Independent that Mr Johnson’s position would be “difficult” if a probe by Whitehall mandarin Sue Gray found against him. The MP said backbenchers were “p***ed off”, adding: “When there’s so much about bending the rules, ignoring the rules – you can only go on for so long.”

       Senior Tory councillors in the north of England have said Mr Johnson must resign if he attended a garden party at the height of lockdown. “If he is found to have attended this gathering, as they call it, he has no option but to resign,” Alan Marshall, a cabinet member with Darlington council, told The Independent.

       However, senior Tory MP Huw Merriman has said that Mr Johnson does not need to resign if he attended a drinks party in May 2020.

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       Asked on Radio 4 Today’s programme if the PM should step down if he would have to step down, Merriman said: “No, I don’t believe so. I do think it needs clearing up. But if you’re saying, ‘Is that a resigning moment’, I don’t believe it is.”

       Deputy Labour leader Angela Rayner said that Mr Johnson’s position would be “untenable” if it is proved that he had attended parties in contravention to lockdown rules. She said the Metropolitan Police had an “obligation” to investigate if lockdown rules were broken.