British Prime Minister Boris Johnson is facing down calls to resign after admitting he really was at a “bring your own booze” party at Downing Street during the first coronavirus lockdown.

Mr Johnson has apologised for being at the gathering on May 20, 2020 when other British people were allowed to meet only with one other person outside.

But he insisted his actions were “technically within the rules” as he believed the party was a “work event” to thank his staff.

“I know the rage they feel with me over the government I lead when they think that in Downing Street itself the rules are not being properly followed by the people who make the rules,” an ashen-faced Mr Johnson told parliament.

“I went into that garden just after six on the 20th of May 2020 to thank groups of staff before going back into my office 25 minutes later to continue working.

“With hindsight, I should have sent everyone back inside.”

The Tory, who won a landslide 2019 election victory on a promise to secure Britain’s exit from the European Union, now faces pressure even from within his own party.

The BBC reports that, shortly after his apology, Mr Johnson was seen “battered and crestfallen” in the Commons tea rooms trying to shore up support from his backbenchers.

British people were in lockdown while there were parties at Downing Street. Photo: Getty

A leadership challenge could be triggered if 54 MPs send letters to the 1922 Committee, an influential group of Conservative backbenchers that meets weekly to discuss their views on the party.

Some of Mr Johnson’s own Conservative MPs have said how he responded to the growing furore would determine whether he could remain in office.

“His survival is in the balance at the moment,” one senior Conservative MP said, speaking on condition of anonymity due to the sensitivity of the situation.

Scottish Conservative leader Douglas Ross said he would write to the committee because he believed the PM’s position was “no longer tenable”, the BBC reports.

Boris Johnson looked “battered and crestfallen”, other MPs said. Photo: Getty

Two snap opinion polls on Tuesday showed well over half of respondents thought Mr Johnson should resign.

Opposition leader Keir Starmer said Mr Johnson must resign and that the public thought he was a liar.

“The party’s over, Prime Minister,” Mr Starmer told him in parliament.

“The only question is: Will the British public kick him out? Will his party kick him out? Or will he do the decent thing and resign?”

-with AAP