LONDON — The British police have opened an investigation into parties held at 10 Downing Street and other government offices during the coronavirus lockdown, an ominous development for Prime Minister Boris Johnson, who is trying to stave off a career-threatening challenge to his leadership over his handling of the scandal.

The commissioner of the Metropolitan Police, Cressida Dick, confirmed on Tuesday that the police are investigating “a number of events that took place at Downing Street and Whitehall in the last two years in relation to potential breaches of Covid-19 regulations.” She declined to give further details.

The disclosure could delay publication of the most serious findings of another crucial investigation, which is being led by a senior civil servant, Sue Gray. Mr. Johnson has appealed to Conservative lawmakers and the public to withhold judgment until the release of that report.

It remained unclear when and in what form Ms. Gray’s report would be published though it could still emerge in the coming days.

On Tuesday morning Downing Street said its findings relating to those parties serious enough to be investigated by the police would be withheld pending their inquiries. But it later said that it was not seeking to block publication and that discussions were underway between Ms. Gray’s team and the police on what could be made public.

The involvement of the police raises the possibility that there were serious violations of lockdown rules. Police officers who guard the Downing Street complex are in a particularly good position to monitor the comings and goings of staff members. Among the more sensational disclosures was that a junior aide was dispatched to a nearby store with an empty suitcase to fill with wine bottles.

The steady drip of reports about social gatherings — most recently news that Mr. Johnson’s wife, Carrie Johnson, and staff members threw him a surprise birthday party in June 2020, when such gatherings were forbidden — has seriously damaged the prime minister’s position with the public and in his own party.

An unknown number of Conservative lawmakers have submitted confidential letters calling for a vote of confidence in the prime minister. If the number of letters exceeds 54, Mr. Johnson would face such a vote, which analysts said would cripple his leadership even if he manages to win a majority of the votes.

“I welcome the Met’s decision to conduct its own investigation because I believe this will help to give the public the clarity it needs and help to draw a line under matters,” Mr. Johnson said in Parliament on Tuesday.

His official spokesman said the prime minister did not believe he had broken the law. No mention of the police investigation was made during a cabinet meeting on Tuesday morning, although Mr. Johnson was informed about it before the meeting.

While the latest development could give Mr. Johnson space to breathe as the police investigation unfolds, it banishes any hope that Ms. Gray’s investigations would clear Downing Street of misbehavior and allow it to quickly move beyond the scandal.

It raises the prospect that Ms. Gray has uncovered information not in the public domain, and the possible delay caused by the police investigation means that more leaks could emerge to keep the issue at the top of the news agenda.

Were Mr. Johnson to be questioned himself by the police, it would not be the first time for a prime minister in recent decades. In 2006 and 2007, toward the end of his tenure as prime minister, Tony Blair was questioned twice by the police over allegations that honors had been given to business leaders in exchange for donations.

Jonathan Powell, who served as chief of staff to Mr. Blair, said that investigation was an extreme distraction for the prime minister and his government, even though prosecutors ultimately decided not to bring charges against anyone.

Similarly, the crisis over parties in Downing Street, which has dominated the news for nearly two months, now looks likely to paralyze the government and continue dragging down the Conservative Party in the polls for a while longer.

Mr. Johnson has struggled to focus on the mounting crisis in Ukraine, ceding the stage until recently to his defense secretary, Ben Wallace, who has been the most vocal British official in pushing back on Russia’s provocations.

On Tuesday, after acknowledging the police investigation, Mr. Johnson delivered a robust statement on Ukraine, promising that Britain and its allies would impose “coordinated and severe sanctions, heavier than anything we have done before against Russia,” if President Vladimir V. Putin invaded Ukraine.

The main question for Mr. Johnson now is whether his own lawmakers are willing to wait for the outcome of the police investigation, or whether the latest twist will prompt enough of them to write formal letters of protest to trigger a motion of no-confidence in him.

On Tuesday one Conservative member of Parliament, Michael Fabricant, wrote on Twitter that he was pleased by the development. “Rather better to have a professional investigation than trial by social and mainstream media!” he said.

In Parliament on Tuesday, several other lawmakers also defended him.

Gavin Barwell, who was chief of staff to the former prime minister, Theresa May, wrote on Twitter, however, that Mr. Johnson’s “only hope of getting through this has been to play for time and hope the public’s anger fades.”

He added that, while the investigations had both downsides and upsides for Mr. Johnson, it was uniformly bad news for the Conservative Party. “The longer this scandal drags on,” he wrote, “the greater the risk that it damages not just the Prime Minister’s reputation but the party’s.”