Boris Johnson is braced for the most perilous 48 hours of his premiership, with exasperated Conservative MPs due to see an official report into Downing Street parties that has now triggered a criminal inquiry. The Metropolitan police commissioner, Cressida Dick, announced on Tuesday that her officers are investigating allegations of law-breaking at the heart of government on the basis of evidence unearthed during an inquiry by the senior civil servant Sue Gray. In what appeared to be a hint about what Gray has discovered, Dick said: “I don’t anticipate any difficulty in obtaining the evidence that it is … necessary, proportionate and appropriate for us to obtain in order to get to the right conclusions”. Those found to have breached regulations could be fined. Former No 10 staffers told the Guardian that the police inquiry will uncover evidence which has not yet been submitted to Gray. One senior Tory said the Scotland Yard inquiry was a “different ballgame”, adding: “Officials who don’t tell Sue Gray the whole truth will not hold back from the cops.” Downing Street sources refused to be drawn tonight on whether the prime minister could give the green light for the report to be released as soon as Wednesday, but it is certainly expected within days. A senior government source cautioned it was unlikely to be published in full, saying: “You don’t need to be a lawyer to realise that if there’s an ongoing police investigation, there are constraints on what you can publish.” Gray has interviewed witnesses and examined key evidence including security logs showing who was in Downing Street and when. She is also thought to have been shown photographs of parties though Cabinet Office sources stressed these were unlikely to be included in the report. It is understood the Met’s decision to investigate a number of parties in Downing Street and Whitehall was made on Sunday. Johnson had been informed in advance but opted not to tell his cabinet at their weekly meeting on Tuesday, leaving them to find out as they emerged and triggering consternation. Asked why cabinet ministers were not told, the prime minister’s spokesperson said: “From what I understand it’s important not to pre-empt a police statement on this sort of issue at any point.” Many Tory MPs are awaiting Gray’s findings before deciding whether to add their names to those calling on the 1922 Committee chair, Graham Brady, to trigger a vote of no confidence. One senior MP suggested Tory backbenchers would be likely to come to a “collective decision” when they see how Johnson responds. “Those colleagues that still have a foot in the real world will recognise that there is no world in which a police investigation is anything but very bad,” they said, adding that in any normal workplace those under investigation would be suspended rather than coming to work as normal. A former cabinet minister suggested Brady may already have received 40 letters. The threshold for triggering a confidence vote is 54. If it is reached, a vote would be held within days, and if Johnson lost it, his premiership would effectively be over. His press secretary insisted last week that Johnson would be determined to fight any vote of no confidence, rather than resign. The prime minister continued to adopt a defiant stance on Tuesday, briefing MPs on the crisis in Ukraine and entertaining wavering colleagues at No 10. “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,” he told the House of Commons. His spokesman, asked whether Johnson believed he had broken the law, said: “It’s fair to say he does not.” At cabinet, Johnson hailed the benefits of Brexit and urged his colleagues to be “bold” and “go further and faster in driving forward the government’s reform agenda”. One backbencher said Johnson’s team “have been working hard” in winning over MPs tempted to call for his removal, saying they had been trying to “charm” backbenchers. Another pointed to an “atmosphere of intimidation”, however, after a series of reports in recent days about the heavy-handed tactics of Johnson’s whips and other close allies. On a day of twists and turns at Westminster, it initially appeared the Met’s inquiry would delay publication of Gray’s report. Johnson’s spokesman subsequently suggested just those aspects of the report unrelated to the parties being investigated by the police could be released. “My understanding is that she can publish those elements that are not subject to further work,” he said. But it later emerged the Met had no objections to the report’s publication. Tory MPs lined up to defend the prime minister during an urgent question on the investigation by Labour’s deputy leader, Angela Rayner. The Conservative MP Giles Watling described the urgent question as a “vexatious waste of everybody’s time” but was forced to withdraw his claims by the Speaker, Sir Lindsay Hoyle, who decides urgent questions. Sir Edward Leigh said: “When Europe stands on the brink of war and there is a cost of living crisis can we please have a sense of proportion over the prime minister being given a piece of cake in his own office by his own staff.” The Liberal Democrat leader, Ed Davey, said later: “A sitting prime minister is under police investigation. If he had a scrap of shame, he’d resign. He thinks he’s above the law – but he’s not. And it’s time for him to go.” On Monday ITV News reported that up to 30 staff gathered in the Downing Street cabinet room in June 2020 with a cake and picnic food from Marks & Spencer to sing Happy Birthday to the prime minister and give him birthday cake. The event, which No 10 has denied was a party, was said to have been organised by his wife, Carrie, and briefly attended by the interior designer Lulu Lytle. Seeking to defend Johnson against that allegation on Tuesday evening, one of his close confidants, Northern Ireland minister Conor Burns, claimed the prime minister had been “in a sense, ambushed with a cake”.