Boris Johnson was braced to potentially receive further fines for breaches of coronavirus laws after a justice minister resigned over the "repeated rule-breaking" in Downing Street.
Conservative peer David Wolfson said he had "no option" other than to quit because the scale and nature of the events determined by police to be breaches so far are "inconsistent with the rule of law".
Multiple newspapers carried reports suggesting the Prime Minister could receive further fixed penalty notices after he accepted a fine for attending a birthday party held for him in No 10 during Covid restrictions in June 2020.
The Mirror reported Johnson could receive three more fines for breaching covid regulations.
Downing Street sources said they were awaiting the outcome of the ongoing Scotland Yard inquiry after Johnson conceded more fines could follow, having reportedly attended six of the 12 events under investigation.
The Prime Minister will attempt to move on from the scandal with a major speech on Thursday setting out new plans for the asylum system, including sending individuals more than 4,000 miles to Rwanda for processing.
The fallout after Johnson and Chancellor Rishi Sunak admitted paying fines this week was continuing with further calls to quit.
Conservative MPs Nigel Mills and Craig Whittaker said the Prime Minister's position was untenable after he was found to have broken the rules he set.
However, Johnson's position was safe for the time being, with politicians away from Parliament for the Easter recess and numerous Tory critics arguing for immediate focus to be on the invasion of Ukraine.
Lord Wolfson, a justice minister since 2020, said in his resignation letter to Mr Johnson that he has come to the "inevitable conclusion that there was repeated rule-breaking, and breaches of the criminal law, in Downing Street".
He concluded that had no option but to resign considering "my ministerial and professional obligations to support and uphold the rule of law".
The decision heaped pressure on Dominic Raab, whose Labour shadow Steve Reed pointed out as Justice Secretary is "constitutionally charged with upholding the law but is instead condoning law-breaking" by backing Johnson.
Raab described Lord Wolfson as a "world-class lawyer" whose "wisdom and intellect will be sorely missed" in Government.
Johnson wrote to the peer saying he was "sorry to receive" the resignation, while praising his "years of legal experience".
Earlier, Mills became the first Tory backbencher to publicly call for Mr Johnson to fall on his sword since the fines landed.
The MP for Amber Valley, in Derbyshire, told the PA Johnson's position was untenable, saying: "Yeah, I think for a Prime Minister in office to be given a fine and accept it and pay it for breaking the laws that he introduced... is just an impossible position."
Call for Johnson and Sunak to resign
Whittaker, the MP for West Yorkshire constituency of Calder Valley, called for both the Prime Minister and Sunak to resign in response to questions from voters.
"To be very clear, my personal opinion is that he and the Chancellor both should resign because you can't set the laws and then break them as they have," he said in a Facebook video.
But Whittaker said he would not be submitting a letter to the 1922 Committee of backbench Tories, saying he expects the Prime Minister would win the vote which he argued would distract the Ukraine and cost-of-living crises.
Welsh Secretary Simon Hart indicated the Prime Minister would not resign even if he was fined multiple times in the Metropolitan Police's Operation Hillman probe.
Hart told Times Radio: "I don't necessarily see the difference between one or two (fines), for example, the principle is the same.
"I personally don't think that for people in public life - or any other walk of life, for that matter - that should necessarily be accompanied by another penalty, which is the removal of your job or similar."
Both Johnson and Sunak - and the Prime Minister's wife Carrie, who was also fined over the party in the Cabinet Room - apologised on Tuesday and confirmed they had paid the fines.
Multiple Tory MPs and Cabinet ministers have expressed their backing for the Prime Minister, pointing to his support for Ukraine in response to Vladimir Putin's invasion.
One who was publicly quiet over the scandal was Home Secretary Priti Patel, but a Home Office source said Johnson has her "full support".
It was argued it was difficult for Home Office ministers to comment on ongoing police investigations.
More than 50 fines have been referred to the Acro Criminal Records Office since the Met's inquiry started.
Speaking to broadcasters at his country residence, Chequers, on Tuesday, Mr Johnson said it "did not occur" to him at the time that the party for which he was fined might be breaching Covid rules.
Sunak offered an "unreserved apology", saying he understood that "for figures in public office, the rules must be applied stringently in order to maintain public confidence".
A spokesperson for Mrs Johnson said: "Whilst she believed that she was acting in accordance with the rules at the time, Mrs Johnson accepts the Metropolitan Police's findings and apologises unreservedly."
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