LONDON — Prime Minister Boris Johnson of Britain on Wednesday announced major new restrictions to curb the fast-spreading Omicron variant, reversing course on a long-held policy at a moment of acute political peril for him, with his staff accused of flouting the rules by holding an office party during last year’s lockdown.

Mr. Johnson’s decision to adopt a contingency plan he had long resisted — even in the face of Britain’s already high daily rate of infections — underscored the threat posed by the new variant. But the timing raised questions about whether Mr. Johnson was motivated more by public health or politics.

Critics, including a lawmaker from his own Conservative Party, accused the prime minister of trying to deflect attention from an outcry over whether officials held a holiday gathering that flouted lockdown rules in December 2020, when Britain was caught in the grip of an earlier coronavirus variant.

Under the government’s new guidance, known as Plan B, people in England will be urged to work from home if possible, starting next week. They will also be required to wear face masks in cinemas, theaters and most other indoor places, and to show a pass proving their vaccinated status to be allowed into nightclubs and large venues, like sports stadiums. He also said that negative tests would be included on vaccine passes as an alternative to proof of vaccination.

“It’s become increasingly clear that Omicron is growing much faster than the previous Delta variant, and is spreading rapidly throughout the world,” the prime minister said. “We need to go further and faster still.”

Mr. Johnson delivered the news Wednesday evening in Downing Street’s briefing room, its wood-paneled walls and Union Jack flags lending gravity to his words. But the same room was featured in a less dignified video that surfaced on Tuesday evening in which Mr. Johnson’s former press secretary, Allegra Stratton, and other aides joked about whether an illicit party had been held in Downing Street during the lockdown.

That exchange, caught during the videotaping of a mock news conference last year, was deeply damaging to Mr. Johnson because it suggested that his aides ignored the very rules they were imposing on the public. They were mocking coronavirus rules at a moment when Britons had been ordered to cancel parties and family reunions, and when some were prevented from saying last farewells to dying relatives.

The smug tone of the mock news conference added to the outrage. At one point Ms. Stratton can be heard saying “Is cheese and wine all right?” and “This fictional party was a business meeting.” She then laughs and adds, “And it was not socially distanced.”

Ms. Stratton, who the prime minister had hired last year to conduct daily White House-style news briefings, resigned as an aide on Wednesday, offering a tearful apology for her remarks, which she acknowledged “seemed to make light of rules, rules that people were doing everything to obey.”

For Mr. Johnson, who has struggled to chart a consistent course from the start of the pandemic, imposing new restrictions amid this outcry will be challenging. The prime minister has often seemed lackadaisical about the need for restrictions, neglecting to wear a mask when visiting hospitals. Allegations of a double standard have long attached themselves to his government.

Last year, his former chief adviser, Dominic Cummings, violated lockdown rules by driving 260 miles to see his parents in the north of England — and then visiting nearby Barnard Castle — at a time when people were being told not to leave home. Mr. Johnson initially stood by Mr. Cummings before cutting him loose months later.

Mr. Johnson denied that he was acting to deflect bad headlines. He tried to keep the focus on the medical data, which he said had encouraged Britons in the past to comply with restrictions on social distancing and vaccinations. There are now 568 cases of the Omicron variant in the country, he said, and it was doubling every two to three days.

Unless its spread is slowed, Omicron could result in between 1,000 and 2,000 hospitalizations a day, according to recently-published minutes from a meeting of the government’s scientific advisory committee, SAGE. Mr. Johnson also hinted that he was thinking about mandatory vaccination, saying there should be a “national conversation” about the issues.

Public health experts welcomed Mr. Johnson’s tougher measures, even if some said they were overdue. But they said the government’s credibility gap cause by the furor over the office party would hamper its efforts to encourage compliance in the public.

“There’s no trust in leadership right now, which is just a major blow during a pandemic when trust is vital,” said Devi Sridhar, head of the global public health program at the University of Edinburgh. She said there was “a fuzzier line between what’s true and what’s false, which again is a death blow to public health messaging and response.”

Downing Street has denied that a Christmas party took place but has not denied that an event of some kind took place. Mr. Johnson has insisted that any gathering that occurred followed Covid protocols.

At his weekly question-and-answer session at Parliament Wednesday, Mr. Johnson apologized for the video but said he was repeatedly assured that no party took place. He said the cabinet secretary, Simon Case, would investigate and that if there were breaches of lockdown rules there would be disciplinary action.

But Mr. Case’s remit does not appear to extend to investigating reports of other parties in Downing Street last year including one that the Daily Mirror claims Mr. Johnson himself spoke at.

The leader of the opposition Labour Party, Keir Starmer, evoked Queen Elizabeth II as he drew an unflattering comparison to Mr. Johnson. “Does the prime minister think he has the moral authority to lead and ask the British to play by the rules?” he asked, as Mr. Johnson grimaced.

Tim Bale, a professor of politics at Queen Mary, University of London, said the contrast between the prime minister’s reaction and Ms. Stratton’s swift, emotional resignation statement might have worsened things for Mr. Johnson, “because it shows him up as someone who refuses to accept responsibility.”

“He is quite a Houdini and predictions of his imminent demise may well be unfounded,” Professor Bale said, “but I do think this is as much trouble as he’s been in politically since he became prime minister.”

Indeed, a week of negative headlines over the party saga appears to have dented trust in Mr. Johnson, according to one opinion survey. Asked by Opinium Research if they thought Mr. Johnson was telling the truth about what happened, 63 percent of respondents said he was not, compared with just 12 percent who believed him. More than half of those polled thought Mr. Johnson should resign.

Amid growing pressure on the prime minister, even some of his own lawmakers appealed publicly for him to get his story straight. One conservative lawmaker, Roger Gale, evoked Mr. Cummings’s downfall, writing in a Twitter post that the crisis bore “all the hallmarks of another ‘Barnard Castle’ moment.”

Worse for Mr. Johnson is the disquiet among his own allies in Parliament where there is a noisy faction that opposes new restrictions.

On Wednesday, before the prime minister announced the new measures, William Wragg, a Conservative lawmaker, said Covid passes “will not increase uptake of the vaccine but will create a segregated society,” and asked Mr. Johnson whether he was aware that “very few will be convinced by this diversionary tactic.”

As weary Londoners contemplated another Christmas compromised by Covid, there was a mixture of anger and resignation at the latest allegations that government officials had yet again violated the rules.

“I stick to the rules, mainly, but I do find it a bit hypocritical,” said Robert Morrissey, 39, who was selling air-dried beef in front of Kings Cross Station. “It’s one rule for them, and one rule for us.”

Munther Mohammed, 32, a Ph.D student based in Cardiff, Wales, said, “Sometimes I feel like as soon as people see the government flouting the rules, they will also flout them — which is disappointing.”

Saskia Solomon contributed reporting.