The highly critical 12-page document was published online an hour before Prime Minister Boris Johnson was scheduled to address lawmakers in Parliament.

“A number of these gatherings should not have been allowed to take place or to develop in the way that they did,” Sue Gray, the civil servant who led the inquiry, said in the report’s conclusion.

Ms. Gray said that law enforcement authorities would have to assess whether Downing Street employees had broken the law. But she wrote repeatedly that the gatherings should not have taken place.

The police confirmed they had asked for critical details to be withheld from the report that was delivered on Monday. By scrubbing the most incriminating material, the report, compiled by a senior civil servant, Sue Gray, could give Mr. Johnson a chance to regroup rather than face a crippling no-confidence vote.

The development outraged opposition leaders and other critics. Some accused the police of an ill-timed intervention that allowed Mr. Johnson to escape judgment for flouting the rules he had imposed on others, as well as for dissembling about it afterward.

“Britain faces huge challenges as we emerge from the pandemic and it is offensive that the government’s sole focus is on cleaning up after themselves,” said the leader of the Labour Party, Keir Starmer.

Still, Mr. Johnson faces a political landscape strewn with land mines. The Gray report could yet be damaging, even in a redacted form. And the police could impose fines on several people for breaching lockdown restrictions, including the prime minister himself.

After a lifetime toiling in anonymity, it is an awkward position for Ms. Gray, who now finds herself famous enough to be the subject of memes, poems and YouTube videos.

Ms. Gray is experienced in solving thorny problems and has a reputation as a steely and skillful fixer. She has spent most of her career in government service although at one point she took a break to run a pub in Northern Ireland with her husband, Bill Conlon, a country singer.

Later Ms. Gray worked closely with Gus O’Donnell, a former head of the civil service. Because of the power of his position, as well as his initials, he was nicknamed “God” and, perhaps inevitably, Ms. Gray was known as “baby God.”

One former minister, David Laws, recalled in his memoirs being told by Oliver Letwin, a fellow minister, that it had taken him two years to realize who really ran Britain. According to this account, Mr. Letwin concluded that it was “a lady called Sue Gray,” adding that “unless she agrees, things just don’t happen.”

Experts have said that Ms. Gray is in an unenviable position for a member of a division that prides itself on its political neutrality. She had to produce a report that is seen as credible, while knowing at the same time that her findings could determine whether enough Conservative lawmakers write formal letters of protest to trigger a no-confidence vote in Mr. Johnson.

Jill Rutter, a senior research fellow at U.K. in a Changing Europe, a research institute, said before the report was released: “She won’t want to open the civil service to the charge that they put a bullet through the prime minister’s forehead, because that would be very bad for our system of government.”

Mr. Johnson then had to admit having attended a gathering in a garden at Downing Street, where the prime minister lives and works, in May 2020 to which 100 people were invited and urged to “bring your own booze.” At that time, official restrictions prohibited socializing with more than one other person, even outside, in almost all circumstances.

He said that he had stayed 25 minutes and that he regretted not ending the gathering of several dozen people who were eating and drinking. He insisted that he had thought he was attending a work event.

The prime minister suffered another damaging blow after reports emerged that staff members had used a suitcase to bring alcohol from a nearby store into Downing Street on the evening before the funeral of Prince Philip, the husband of Queen Elizabeth II. Mr. Johnson did not attend the gathering, and later apologized for two parties that took place at Downing Street that evening.

At one event, a disco was set up in the basement, and the swing of Mr. Johnson’s toddler son was broken as staff members drank into the early hours, according to news reports.

Then, last week, the broadcaster ITV reported that Mr. Johnson had a 30-person surprise birthday lunch in Downing Street’s cabinet room in June 2020. His office told ITV that he had spent “less than 10 minutes” at a brief gathering organized by the staff, and one loyalist came to Mr. Johnson’s defense.

“He was, in a sense, ambushed with a cake,” said Conor Burns, a minister and ally of Mr. Johnson.

On Jan. 25, the British police said they had opened an investigation into “a number of events” at Downing Street and other government offices during lockdowns, “in relation to potential breaches of Covid-19 regulations.”

The investigation was prompted by information turned over by the inquiry led by a senior civil servant, Sue Gray, whose investigation into parties that might have breached lockdown rules was delivered to Mr. Johnson on Monday.

Although its findings could inflict further damage on Mr. Johnson, the report was expected to be heavily redacted after the police confirmed last week that they had asked for critical details to be withheld to avoid prejudicing their own inquiry. A spokesperson for Mr. Johnson’s cabinet confirmed that he had received “an update” on the investigation, signaling that much of the report was redacted.

The report is expected to be published later on Monday. Mr. Johnson was studying the report and was expected to make a statement to Parliament in the afternoon.

Still, the details in the report, prepared by a senior civil servant, Sue Gray, could prompt Conservative Party lawmakers to trigger a no-confidence vote that could eject Mr. Johnson from power. That, in turn, raises the extraordinary prospect of the prime minister being forced out a little more than two years after winning a landslide election victory for his party in December 2019.

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