LONDON, Jan 18 (Reuters) - British Prime Minister Boris Johnson denied on Tuesday an accusation by his former adviser that he had lied to parliament about a lockdown party, saying nobody had warned him the "bring your own booze" gathering might contravene COVID-19 rules. Johnson faces the gravest crisis of his tenure after revelations about gatherings during lockdowns, some when British people could not even bid farewell in person to dying relatives and the Queen was mourning her husband. Propelled into the top job to "get Brexit done", Johnson won his party"s biggest majority in more than 30 years but now faces calls to resign from opponents and even some of his own lawmakers. read more Asked if he had lied to the public and parliament, Johnson told reporters: "No. Nobody told me that what we were doing was, as you say, against the rules ... I thought that I was attending a work event." Johnson sidestepped questions about whether he would resign if proven he misled parliament, saying only that he wanted to wait for the outcome of an internal inquiry. "He’s the prime minister, he set the rules, he didn’t need anyone to tell him that the party he attended broke them," said Angela Rayner, deputy leader of the opposition Labour Party. "If he had any respect for the British public, he would do the decent thing and resign." Johnson used the short interview during a visit to a hospital to apologise for mistakes made in Downing Street, including for parties held by staff on the eve of the funeral of Prince Philip, Queen Elizabeth"s husband. "I deeply and bitterly regret that happened and I can only renew my apologies both to Her Majesty and to the country," he said. Johnson had already last week apologised to parliament for attending the May 20, 2020 gathering in the Downing Street garden. He said he was there for 25 minutes to thank staff. But Dominic Cummings, an architect of Britain"s departure from the European Union and a former senior adviser who left government under acrimonious terms in November 2020, said that Johnson had agreed the drinks party should go ahead. Cummings said that he and at least one other adviser told Principal Private Secretary (PPS) Martin Reynolds, the official who invited people to the party, that it should not go ahead. The warning was sent via email, and Reynolds checked with Johnson if the event should go ahead, according to Cummings. "I told the PPS the invite broke the rules ... The idea that the PPS would be challenged by two of the most senior people in the building, say he’d check with the PM then not — is not credible," Cummings said in his blog. "The PM agreed it should (go ahead)," Cummings added. "The events of 20 May alone, never mind the string of other events, mean the PM lied to parliament about parties." Senior civil servant Sue Gray is investigating about a dozen allegations of rule-breaking by Johnson, his team and officials at 10 Downing Street. Cummings told Sky News he will be questioned by her investigation. Senior ministers have said people needed to wait for the conclusion of her inquiry. However, the scandals have seen his and the Conservative Party"s ratings plummet.