Front pages of British newspapers are displayed outside a newsagent, in London, Saturday, Jan. 15, 2022. AP
In the latest, Johnson's wife Carrie was photographed on the front page of the Sunday Telegraph newspaper embracing a friend at a September 2020 party, in apparent violation of the then rules on social distancing.
At least six lawmakers in Johnson's Conservative party have now called publicly for him to quit, while others say they are awaiting the findings of an internal inquiry by senior civil servant Sue Gray.
Labour leader Keir Starmer, who is enjoying an opinion poll surge on the back of the Downing Street "partygate" allegations, said the facts were already clear.
"I think he broke the law. I think he's as good as admitted that he broke the law," Starmer told BBC television.
Gray only has the remit to establish the facts, he added, calling for the police to look into a possible criminal inquiry once her report is out.
"The prime minister has degraded the office of prime minister and he has lost full authority not only in his own party, but in the country," Starmer said.
Government minister Oliver Dowden, the Conservatives' co-chairman, conceded that the Downing Street parties were "completely unacceptable".
But Dowden backed Johnson to stay in office, while stressing he would take "full responsibility" once Gray releases her findings.
"I can tell you that when he responds to the House of Commons, as he has committed to do so, he will make sure that we address the kind of culture that has allowed that to happen in the first place," he told Sky News.
Conservative MPs, many of whom were spending the weekend back in their home constituencies, say they are being deluged with messages from voters outraged at the accounts of rule-breaking in Downing Street.
After weeks of denials and stonewalling, Johnson this week apologised in parliament for at least one boozy event organised by his staff which he attended in May 2020, when Britons were banned from socialising.
Two other parties were held in April 2021 as Queen Elizabeth II prepared to bury Prince Philip, her husband of 73 years. Downing Street sent apologies to Buckingham Palace, calling them "deeply regrettable".
But those were not isolated events, according to Saturday's Daily Mirror, which published a photograph of a fridge being delivered to a Downing Street back door in December 2020 for "Wine time Fridays".