LONDON — The scandal over lockdown-breaking parties that has ensnared Prime Minister Boris Johnson of Britain took a new turn on Monday when his rival, the opposition leader Keir Starmer, promised to resign if police decide that he also broke coronavirus laws.
The statement followed days of speculation about whether Mr. Starmer was in breach of the country’s strict Covid-19 regulations when he was pictured drinking a beer as he ate takeout Indian food during a campaign meeting in the northern city of Durham last year.
And while Mr. Starmer’s promise represents a striking gamble, it also raises the pressure on Mr. Johnson, who has already been fined for attending a birthday celebration at Downing Street, but has refused to quit.
“I am absolutely clear that no laws were broken, they were followed at all times,” said Mr. Starmer, who is a former chief prosecutor. “But if the police decide to issue me with a fixed penalty notice, I would of course do the right thing and step down,” he added, referring to the mechanism used by police to fine those deemed to have broken the law.
In recent days, the drama swirling around Mr. Starmer, dubbed “beergate” in the British media, has delivered a significant setback to the opposition leader, overshadowing the progress his opposition Labour Party made in local elections last week. Labour is ahead of Mr. Johnson’s Conservatives in the opinion polls and, if the tilt toward his party was to be replicated at the next general election, Mr. Starmer would stand a reasonable chance of becoming the country’s next prime minister.
Mr. Starmer insists he did not break the rules, and that was also the official verdict of an initial police investigation. But on Friday, following a succession of media reports and after receiving a new complaint, Durham police said they would reopen the investigation, citing new evidence.
Since then Labour has been on the defensive, despite its strong performance in local elections, while trying to dismiss the furor as a smear. But Mr. Starmer has called on Mr. Johnson to quit over the lockdown-breaking parties at Downing Street.
As well as demanding Mr. Johnson’s resignation, Mr. Starmer has also said that Rishi Sunak, the chancellor of the Exchequer, should resign after he was fined by police for his brief attendance at a celebration for the prime minister’s birthday at Downing Street.
Conservative-supporting newspapers have accused Mr. Starmer of hypocrisy, and he appears to have calculated that, if the police fine him, his position would be untenable anyway.
It is still unclear whether Mr. Starmer’s actions in Durham broke the law.
Adam Wagner, a lawyer and expert on the coronavirus rules, wrote on Twitter that he thought it was “absurd” that the police could criminally penalize the leader of the opposition for meeting, for an hour, with a local lawmaker “and having dinner late in the evening — days before an election in that area.”
Understand Boris Johnson’s Recent Troubles
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Turmoil at Downing Street. A steady drip of disclosures about parties that violated lockdown rules has ensnared Prime Minister Boris Johnson of Britain in a scandal that could threaten his hold on power. Here is what to know:
Contentious gatherings. The British news media reported that staff members were invited to a party in the backyard of Mr. Johnson’s residence in May 2020, when officials were instructing people not to socialize to curb the spread of the coronavirus. Subsequently, details about other gatherings involving government officials that violated Covid rules have emerged.
What’s at stake. The crisis has stoked speculations that the political future of Mr. Johnson might be at risk. Though few Conservatives in Parliament have publicly called on him to quit, if the investigation determines that he misled Parliament, it could cost him his job.
Labour argues that the event took place at a time when many restaurants were closed but work meetings and political campaigning were permitted, and that Mr. Starmer and his aides had no alternative but to order a takeout meal. The party’s story changed in one respect, however, when it admitted that its deputy leader, Angela Rayner, was also at the meeting, contradicting earlier denials.
Ms. Rayner also said Monday she would step down if she was fined.
A police investigation into several allegations of lawbreaking at Downing Street is still continuing. And once it is completed, the government is committed to publishing an internal report, a preliminary version of which was critical enough to prompt the police investigation.
There are signs that some of the prime minister’s allies are becoming concerned that the “beergate” saga might have spiraled out of control.
Asked on Sunday whether Mr. Starmer should quit if he is fined, one senior cabinet minister, Jacob Rees-Mogg, said he should not. “I think he should pay a fine and then talk about the issues of great importance to the nation,” he told Channel 4.