Boris Johnson suffered a fresh blow over the “partygate” scandal on Wednesday when his justice minister quit over “repeated rule-breaking and breaches of the criminal law” in Downing Street.

Lord David Wolfson said he could not remain a minister following the revelations about Number 10 parties held during coronavirus lockdowns.

Wolfson’s resignation came as senior Tories expressed concern the scandal has much further to run, with the prime minister facing the possibility of more fines over Downing Street parties and the risk of photographs of gatherings being leaked to the media.

Johnson became the first UK prime minister to have a committed a criminal offence while in office after the Metropolitan Police issued him with a fine for attending a surprise Downing Street birthday party in June 2020. Chancellor Rishi Sunak also received a fixed penalty notice over the same event.

Johnson and Sunak on Tuesday said they had paid the fines and apologised.

Sunak’s allies played down a media report that he had spent seven hours “agonising” on Tuesday about whether to resign after receiving his fixed penalty notice.

Wolfson, who was appointed a junior minister in the Ministry of Justice in December 2020, said he had “no option” other than to resign as he criticised the “official response” to the partygate revelations.

He told Johnson in a letter: “Justice may often be a matter of courts and procedure, but the rule of law is something else — a constitutional principle which, at its root, means that everyone in a state, and indeed the state itself, is subject to the law.

“I regret that recent disclosures lead to the inevitable conclusion that there was repeated rule-breaking, and breaches of the criminal law, in Downing Street.”

A handful of Conservative MPs called on Johnson to resign on Wednesday. Nigel Mills, a Tory backbencher, said the prime minister’s position was “untenable”. Caroline Nokes and Gary Streeter, two former ministers who have previously called for Johnson to quit, restated their positions.

But transport secretary Grant Shapps defended Johnson, saying he “didn’t knowingly break the law”. He added the prime minister was “mortified” and “incredibly embarrassed” at having been fined by the police.

The Met has so far issued more than 50 fines to an unknown number of individuals as part of its investigation into 12 different government parties in 2020 and 2021 which potentially breached Covid-19 restrictions.

Senior Conservative officials said they were worried that with the Met yet to complete its probe, Johnson could be hit with further fines in the run-up to local elections on May 5.

They also expressed concern about the risk that media organisations are leaked some of the estimated 300 photographs, relating to gatherings in Number 10 and Whitehall, that were handed over by the government to the Met.

One Whitehall insider said he had seen a photograph of several government figures — apparently drunk — sat around a table inside Downing Street late at night during one of the Covid lockdowns.

“If and when the public get to see photographs of people drinking booze at Number 10 parties then you could see another wave of anger — images have a much more visceral effect than hundreds of words of prose,” said one former Downing Street official. “People are very worried that the pictures could still come out.”

One senior Conservative MP said the partygate saga could drag on for weeks or even months, partly because a final report into the gatherings commissioned by Johnson and being prepared by senior civil servant Sue Gray is not expected to be released until after the local elections.

The MP added: “Fines will continue to be issued running into polling day, which could make the locals worse. Then we have to face the [publication of the] full Sue Gray report. That is the point Johnson faces maximum danger as it may all prove too much.”

Meanwhile Sunak’s allies said he had been stung by what he believed to be an “unfair” fine over Johnson’s June 2020 birthday party, and that he had been considering his political future after intense scrutiny of his wife’s UK tax perks because of her non-domiciled status.

Some of Johnson’s allies have urged the prime minister to move Sunak in a future cabinet reshuffle, saying the row over his family’s tax affairs and his much criticised Spring Statement had made him an electoral liability.

One Johnson supporter said: “Rishi has proven himself to be crap at politics over the last couple of weeks. I don’t think he has good enough political judgment to last.”

But other allies of the prime minister said Johnson and Sunak, following tensions over the partygate scandal, had been “brought closer together” by the fact they had both received fines. “The fact Rishi didn’t resign means they’re in it together,” said one ministerial aide.