Pressure is intensifying on the Metropolitan police as they face a formal call to justify letting Boris Johnson escape a fine for drinking at a social gathering during lockdown, with some senior officers bewildered by the force’s handling of the Partygate inquiry. The prime minister will be forced to explain his presence at the event when Sue Gray’s Whitehall investigation that has hung over his premiership for seven months is finally published. Tory MPs are expected to use the top civil servant’s report, which could come on Wednesday, to decide whether to force a leadership contest or accept Johnson’s apology once and for all. On Tuesday evening new pictures emerged in the Mirror of an event in No 10 apparently not investigated by either the Met or by Gray – raising further questions about the robustness of the police inquiry. Scotland Yard last week closed its investigation into Partygate, leaving Johnson with one fine – despite claims he was an active participant at several gatherings that breached his own government’s Covid rules. On Monday that decision came under scrutiny and widespread criticism when photographs emerged of Johnson with a glass in his hand, surrounded by other people clutching glasses, apparently toasting the departing Downing Street aide Lee Cain in front of a table littered with empty bottles of alcohol. Even inside the Met there is incredulity that the photo did not lead to a fine for the PM, and concern about the decision making. One source, a staunch defender of the Met, said: “There’s a lot of questions to answer. “I’m baffled … I think it is blindingly obviously evidence of a breach. The least you would do is question the individual. If I had been responsible for the investigation I’d have to be convinced why the photo did not show a breach.” Another source, again a staunch defender of the Met and veteran officer, said: “I’m looking at that photo and wondering, why is that not an FPN [fixed-penalty notice]?” The Met said it would provide no further explanation beyond a statement it made last week. For the Met, the strength of the evidence gathered was a “crucial” fact – it had to be strong enough to ensure the case could be prosecuted at court in the event the decision to issue a fixed penalty was contested. Before issuing fines, police considered each element of the offence under the regulations: “Each line of the inquiry looked at the date, circumstances behind each event” and “the actions of each individual benchmarked against the legislation at the time to establish whether the behaviour met the criminal threshold for an FPN.” Police looked at how many people were at the gathering, whether the gathering was covered by an exception and, if not, whether the individual had a reasonable excuse for participation in that gathering. Another source added that questionnaires police sent to junior staff were largely fully answered with many admitting fault and thus receiving penalties, but more senior staff were said to be less forthcoming. Downing Street staffers told the BBC there had been a “witch-hunt” of junior officials over the parties, who “did not think they were breaking the rules at the time because the prime minister was at them, some of the most senior civil servants in the country were at them”. They also said the event attended by Johnson was so crowded some people were forced to sit on each other’s laps, and that some mornings they would walk into the office to find empty bottles from parties the night before. Amid the furore, Sadiq Khan announced on Tuesday he had written to the acting Met commissioner, Sir Stephen House, asking that Britain’s biggest force explain to the public how decisions were reached. Khan, as well as being mayor of London, oversees the Met as the police and crime commissioner for the capital. He said he was concerned that trust and confidence in the police was being lost as incredulity at the decision grew after the photos were leaked to ITV News. A City Hall spokesperson said: “He has asked them to take steps to also reassure Londoners by making this explanation to them directly, because he is concerned that the trust and confidence of Londoners in the police is being further eroded by this lack of clarity. “The mayor has been clear he cannot and would not intervene in operational decisions, however with the investigation now complete, he has made this request in accordance with the Policing Protocol Order 2011 paragraph 23(g).” The protocol says “the chief constable is responsible to the public and accountable to the PCC” and adds the chief is responsible for “notifying and briefing the PCC of any matter or investigation on which the PCC may need to provide public assurance either alone or in company with the chief constable”. It also says the chief constable should remain “politically independent of their PCC”. When Gray’s report is published, Johnson will hope to draw a line under the scandal. He is expected to make a statement to the Commons, followed by fielding questions at a private meeting with Tory backbenchers, and potentially a press conference. But Mark Harper, the Conservative MP and former chief whip who has called for Johnson to quit, said he was fed up with “decent” colleagues “being asked to go out on the television day after day and saying things that are frankly ridiculous and defending the indefensible”. He added the prime minister had not been straightforward with people and that senior ministers could not give straight answers about basic questions. Another Tory backbench critic of Johnson’s, Roger Gale, said the prime minister had misled the Commons by denying any gathering was held in November 2020. He said: “That is a resignation issue. I have made my own position clear. It is now a matter for my Conservative parliamentary colleagues to decide whether or not to instigate a vote of no confidence.” Gray’s report was not initially expected to contain photos taken at some of the law-breaking parties, but could do when the final version is delivered to No 10. Some insiders believe this will help stymie criticism of it being a whitewash, particularly if such evidence comes out at a later date, for instance during the investigation into whether Johnson misled parliament by the privileges committee. There may also be an attempt to counter the narrative sown by ministers in recent months that such gatherings were brief and frivolous. Referring to the damning photos and video evidence of the parties that has emerged so far, a source said: “A picture paints a thousand words.” The Met has been referred by the Liberal Democrats to the police watchdog, the Independent Office for Police Conduct, about the handling of its investigation. The IOPC referred the complaint to the directorate of professional standards in the Met, who will have 15 days to respond with how they intend to handle it.