Defence minister James Heappey has said he “sincerely hopes” that Boris Johnson did not mislead parliament about the drinks party in the No 10 garden during the first Covid lockdown in May 2020.

       The prime minister is battling to save his premiership, amid reports the 54 letters which would launch a no confidence vote could be received on Wednesday.

       In a message to MPs considering sending a letter to the 1922 Committee of Tory backbenchers, Mr Heappey told Sky News: “This doesn’t feel like the time to be changing prime minister, if you ask me.”

       However, the minister refused to say whether Mr Johnson should resign if he is found to have broken Covid law. “We are all keen to understand exactly what happened and whether it not it was breaking law … it is perfectly reasonable to wait for Sue Gray’s report.”

       Asked on Times Radio if a report by investigating civil servant Sue Gray could find the PM misled parliament, Mr Heappey said: “It may.”

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       He added: “Ministers know that when they stand up at the dispatch box they need to be accurate with their language. I sincerely hope the prime minister was.”

       A seventh Tory MP confirmed he had submitted a letter of no confidence in Mr Johnson, with up to 20 reportedly set to follow suit in a growing mutiny emboldened by the party’s 2019 “red wall” intake.

       A group of angry red wall Tory MPs is understood to discuss Mr Johnson’s downfall on Wednesday in a move dubbed the “Pork Pie Plot” because it allegedly involves Alicia Kearns, the MP for Rutland and Melton – home of the Melton Mowbray delicacy.

       Asked about the rebellion on Sky News, Mr Heappey said: “Some colleagues appear to have met to discuss things. I don’t think that’s surprising – I suspect there are lots of colleagues who are reflecting what they have heard in their constituencies and are feeling under a lot of pressure.”

       The Armed Forces minister said he accepted Tory MPs were “competing loyalties” to the prime minister and their constituents – but urged colleagues to keep “cool heads” and wait for the outcome of Ms Gray’s report.

       He told BBC Breakfast: “I have chosen to believe the prime minister, I hope colleagues do likewise. But I understand there are very many people out there who don’t want to give him the benefit of the doubt, and that’s why it’s right there is an investigation.”

       The minister also suggested Mr Johnson had been let down by his staff, arguing that “the prime minister really doesn’t own his own diary,” adding: “The reality is that those who work around the prime minister need to have his back.”

       Mr Johnson insisted on Tuesday that he wasn’t told it was against the rules for him to attend a gathering in the garden of Downing Street on 20 May 2020. Mr Heappey claimed the PM’s diary is so “congested” he cannot be expected to know what he was attending.

       “I can see how the prime minister wouldn’t have known what it was he was going down the stairs to join … He would have been grabbed from his office, taken downstairs and briefed on what he was going to, as he went,” he told BBC Radio 4’s Today programme.

       Asked about the so-called drinking culture in Whitehall, the defence minister said he saw government staff “very occasionally have a glass of wine together at the end of work”.

       But Mr Heappey added: “I think that there does need to be some reflection about the amount of alcohol that appears to be consumed and how regularly, not just the No 10 but in a number of departments of state.”

       Red wall Tory Christian Wakeford, MP for Bury South, became the seventh Conservative MP to publicly call for Mr Johnson to go on Tuesday.

       Some MPs have suggested the expected flurry of no-confidence letters to the chair of the backbench 1922 Committee, Sir Graham Brady, could take the total to the 54 needed trigger a vote of no confidence as soon as Wednesday. One told the Daily Telegraph: “His time has gone.”

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       One Tory backbencher who is “close” to sending in a no-confidence letter told The Independent that opposition to Mr Johnson was strongest among “one nation” Tories who had been opposed to Brexit, as well as some in the Covid Research Group (CRG) who had been vehemently against restrictions.

       “Many of the 2019 intake feel they owe their seats to him [Mr Johnson]. So if some of them are turning against him it’s a bad sign for him.”