* Johnson denies he was told Downing St gathering broke rules
* Ex-adviser Cummings says PM lied about drinks party
* British leader facing gravest crisis of premiership (Adds Johnson quote)
By Guy Faulconbridge and William James
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 https://www.reuters.com/world/uk/sleaze-scandal-lockdown-parties-trying-times-uk-pm-johnson-2022-01-11 of his tenure after revelations about gatherings during COVID-19 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 https://www.reuters.com/world/uk/how-could-uk-pm-johnson-be-ousted-by-unhappy-lawmakers-2022-01-12 from opponents and even some of his own lawmakers.
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, that the event in question, was, something, that we were going to do something that wasn't a work event ... I thought that I was attending a work event."
Johnson sidestepped several questions about whether or not he would resign if it was proven that he had misled parliament. But he apologised for mistakes made in Downing Street.
Johnson last week also apologised to parliament for attending the May 20, 2020 "bring your own booze" gathering in the Downing Street garden. He said he had attended for 25 minutes to thank staff.
'BROKE THE RULES'
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, according to Cummings.
"I told the PPS the invite broke the rules," he said. "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."
Johnson's apology came after ITV News published an email invitation from Reynolds to the event. Cummings said that after being recommended to cancel the invitation, Reynolds checked with Johnson if it should go ahead.
"The PM agreed it should," Cummings said in his blog. "The events of 20 May alone, never mind the string of other events, mean the PM lied to parliament about parties," he wrote.
Senior civil servant Sue Gray is investigating about a dozen allegations of rule-breaking by Johnson, his team and officials at 10 Downing Street. 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.
(Reporting by Guy Faulconbridge; Editing by Michael Holden and Andrew Cawthorne)
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