Boris Johnson faces a make-or-break session of prime minister’s questions on Wednesday, with furious Conservative MPs awaiting his explanation of the “bring your own booze” garden party in May 2020. Labour’s deputy leader, Angela Rayner, said: “He has an opportunity now to come clean to the British public, who are devastated by these allegations. “What’s astonishing is the prime minister has had a couple of days now where he could have quite simply answered that question yes or no – was you there at this party? – and he’s refused to do that.” More than 10 Conservative MPs have publicly criticised the Downing Street gathering, details of which emerged in a leaked email from Johnson’s principal private secretary, Martin Reynolds. The Amber Valley MP Nigel Mills said Johnson’s position would be “untenable” if he attended the party. “If the prime minister knowingly attended a party, I can’t see how he can survive having accepted resignations for far less,” he told BBC News. “He accepted the resignation of his spokesperson [Allegra Stratton] for not attending a party but joking about it at a time of much lighter restrictions. I just think that’s untenable.” Many more Tory MPs are expressing anger in private, though some are prepared to await the findings of an inquiry by the civil servant Sue Gray into a string of parties in Downing Street. One former minister said Conservatives would be watching prime minister’s questions closely. “Off the back of that, you will get a very clear sense from colleagues about whether they think that is a sustainable position.” Several witnesses have said Johnson and his wife, Carrie, attended the gathering. The Liberal Democrat leader, Ed Davey, called for him to resign. “Millions of people obeyed the lockdown rules, often at huge personal cost. They missed funerals, cancelled weddings and said goodbye to dying loved ones on video calls – some on the very day that No 10 illegally hosted a garden party,” he said. “Boris Johnson has become a threat to health of our nation, and for the sake of the country still gripped by this awful pandemic he must resign.” The Labour leader, Keir Starmer, will emerge from Covid self-isolation to pose questions to Johnson in the House of Commons, after receiving a second negative lateral flow test. He is expected to accuse the prime minister of misleading parliament about his role in lockdown social events. The 20 May party – as well as a gathering on 15 May revealed by the Guardian – took place at a time when the public were allowed to meet outdoors only one person from outside their household. The government did not put forward a minister to answer questions in broadcast interviews on Wednesday morning. On Tuesday, the junior minister Michael Ellis was sent out to take MPs’ questions about the party, insisting most of these must wait for the Gray inquiry. Rayner said: “Of course Sue Gray has an important role in terms of the wider investigation, but we don’t need to wait for that.” She added that the Metropolitan police “could and should” investigate the event. The Met have said they are in discussions with the Cabinet Office about the claims.