Ms Rayner told Sky News the prime minister has "not only broke the rules, but he's lied to the British public" and said the fact Mr Johnson has not yet come forward to explain his account of the event "shows the measure of the man".

The prime minister, who so far has avoided questions over the alleged "bring your own booze" event, will face the House of Commons at midday amid intense pressure from Conservative MPs and Tory donors.

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Both Conservatives and opposition MPs are demanding the prime minister confirms whether he attended a drinks gathering in the Downing Street garden on 20 May 2020.

The Liberal Democrats are calling for the PM to resign, saying he has "become a threat to the health of our nation".

"Boris Johnson has broken the law and lied to parliament and the country, and he must now go," the party's leader Sir Ed Davey said in a statement.

Amis increasing public anger - with two opinion polls on Tuesday suggesting the public was turning on the PM - Wednesday's PMQs is set to be one of the most important Commons moments of Mr Johnson's political life.

A YouGov poll showed 56% of respondents believed Mr Johnson should resign over the fresh "partygate" allegations, with 27% saying he should remain.

And a Savanta ComRes study found 66% of British adults thought he should quit as prime minister, with 24% saying he should stay.