Dominic Cummings has accused Boris Johnson of lying after No 10 denied the prime minister was warned against allowing a “bring your own booze” party during the first lockdown. Johnson admitted to parliament last week that he attended drinks in the Downing Street garden on 20 May 2020 but claimed he had not realised it was a social gathering. The event was organised by Johnson’s principal private secretary (PPS), Martin Reynolds, who told people to “bring your own booze” – but No 10 denies that the prime minister saw the emailed invitation to about 100 staff. It has also denied allegations that two senior staffers warned Johnson not to go ahead with the drinks event, saying this was “not true”. But Cummings, a former senior aide to the prime minister, wrote a new blog post on Monday challenging that account. He said he personally told Reynolds that the invitation broke the rules and claimed Reynolds replied: “So long as it’s socially distanced I think it’s OK, I’ll check with the PM if he’s happy for it to go ahead.” Cummings said in his blogpost: “Not only me but other eyewitnesses who discussed this at the time would swear under oath this is what happened.” Sue Gray, the senior civil servant in charge of the inquiry into Downing Street parties, could make a formal request to interview Cummings, Whitehall sources confirmed. It is within the remit of the inquiry to approach former members of staff if their evidence might be relevant. No 10 staff had hoped that the inquiry, which is examining more than 15 separate allegations of illegal gatherings in Downing Street, would conclude this week. Because of the constant drip of new parties and developments, officials believe it may now report next week. The inquiry is expected to outline when each alleged party occurred, how many people were present and who was involved in organising them. It will also outline the regulations at that time, and could suggest whether each event appears to have broken the regulations at that time. The Gray report is not expected to recommend a criminal investigation. Cummings then said that during a discussion over the future of the cabinet secretary and Reynolds, he had said to the prime minister something like: “Martin’s invited the building to a drinks party, this is what I’m talking about, you’ve got to grip this madhouse.” Cummings added: “The PM waved it aside. I had told him repeatedly the PPS should be replaced, as had other competent officials who knew the whole structure needed a huge upgrade in personnel and management. ‘He’s MY guy, I don’t want you replacing him with YOUR person.’ (Yes, this says a lot.) I went home to bed at 3ish, still very ill from Covid.” He claimed Reynolds had checked with Johnson whether the party should go ahead, the prime minister agreed it should and they both went to the party. Another former Downing Street staffer told the Guardian: “It is inconceivable: there is no way Martin would go ahead without checking with Boris. There is no way any PPS would. If two senior people come to you and say, ‘this shouldn’t happen’, you don’t then proceed with it without speaking to the principal.” Asked about Cummings’s latest claims, a No 10 spokesperson pointed to an earlier statement, saying: “It is untrue that the prime minister was warned about the [20 May] event in advance. As he said … he believed implicitly that this was a work event. He has apologised to the house and is committed to making a further statement once the investigation concludes.” The party on 20 May has emerged as the most contentious event for Johnson out of all the allegations that lockdown rules were broken in Downing Street. With Tory MPs openly questioning Johnson’s future as party leader, there are worries among backbenchers about a drip-drip of further damaging revelations causing harm to the government’s reputation.