Johnson has so far ruled out resignation but several of his own lawmakers have urged him to do so or face a vote of no confidence.
Boris Johnson was again urged to resign by opposition leader Keir Starmer during parliamentary question time on Wednesday after news that Scotland Yard would open its own investigation into parties held during COVID-19 lockdown at Downing Street.
Johnson, who is facing MPs for his weekly Prime Minister's Questions (PMQs), said he could not comment on an ongoing inquiry and - as in previous appearances - urged lawmakers to wait for the results of civil servant Sue Gray's report into the scandal.
In an often unruly parliamentary session, speaker Sir Lindsay Hoyle had to repeatedly intervene to warn MPs over shouting down Starmer during his questions and threatening to eject those that continued to do so.
Johnson responded to Starmer by citing achievements of the Conservative government since he took office and attacking the Labour leader as "a lawyer, not a leader".
But despite his comments, partygate continued to dominate proceedings in parliament with Scottish National Party (SNP) leader Ian Blackford arguing that Johnson "demeans the office of prime minister."
The parties in question include a "Bring Your Own Booze" party on 20 May — on the eve of Prince Philip's funeral — as well as a Christmas party and a birthday gathering for Johnson.
The allegations are already the subject of an internal investigation spearheaded by senior civil servant Sue Gray. According to British media, her report is expected to come out on Wednesday afternoon or Thursday.
Johnson has apologised for the gatherings although he said he thought the 20 May party was a work event and that nobody told him it would breach his government's COVID-19 rules.
But he has for now ruled out resignation despite repeated calls from opposition parties and lawmakers from within his own Conservative ranks.
Angela Rayner, deputy leader of the main opposition Labour party, said on Wednesday morning that this has gone on long enough.
"Politics has real work to do. Boris Johnson has to go or the Conservatives have to make him," she wrote on Twitter.
Ian Blackford, the Westminster leader for the Scottish National Party, told Sky News on Wednesday that Johnson "needs to recognise he's lost the trust of the public, he's demeaned the office of Prime Minister."
"This is embarrassing, it's a stain on our democracy," he said, adding: "He really needs to recognise that he has lost the right to govern."
"Those in the Tory party must take their responsibility — show this charlatan the door," he also said.
Several Conservatives MPs have already sent or drafted a letter to the party's 1922 Committee to request a vote of no confidence against their leader. Others are said to be waiting for Gray's report to make their decision.
A total of 54 letters — or 15% of the Conservatives' 359 MPs — are needed to trigger a vote that could lead to Johnson's ouster and to a leadership contest.
The UK is Western Europe's worst impacted country by COVID-19 with more than 154,000 deaths.