Conservative MPs fear a “cover-up” over potentially the most damaging event of the Partygate scandal after Sue Gray admitted she did not fully investigate an alcohol-fuelled gathering in the flat shared by the prime minister and his wife. The six-month inquiry concluded with an acknowledgment from Gray that little was known about what took place in the flat above 11 Downing Street on 13 November 2020, with food, alcohol and loud Abba music reported. Gray said it would not have been “appropriate or proportionate” to continue her inquiries into the gathering after they were paused to make way for a Scotland Yard investigation. Her report said a “meeting” was held in the Downing Street flat involving Johnson and five political special advisers to discuss the resignations of two senior No 10 aides that day – Johnson’s chief of staff, Dominic Cummings, and the director of communications, Lee Cain. Gray said the meeting began after 6pm and Johnson joined at about 8pm, and the discussions “carried on later into the evening” with food and alcohol available. But she admitted her knowledge of the gathering was limited because she had only just started collecting evidence about it before the Metropolitan police announced its own Partygate investigation in January, prompting her investigation to stop to avoid prejudicing officers’ inquiries. When the Met’s Operation Hillman came to an end last week, with 126 fines handed out, Gray said she “considered whether or not to conduct any further investigation into this event but concluded it was not appropriate or proportionate to do so”. One senior Tory MP told the Guardian they believed it amounted to a cover-up. Another said it had the potential to be “the most damaging event of the bunch for Johnson personally” and suggested it was highly suspicious the event had not been looked at, given several of the people present are believed to be friends of Johnson’s wife. A frontbench Conservative MP also said they were disappointed the Gray report “doesn’t clear up what parties did or didn’t happen in his flat”, and added: “I think he’s getting away lightly.” Another Tory MP: “The report makes clear the PM attended party after party in his frat house. “While he partied in his, others were domestically abused or isolated in theirs. The failure to investigate the infamous Abba party is a failure of courage and duty on the part of Gray.” Chris Bryant, a Labour MP and chair of the Commons standards committee, said he was “mystified why this hasn’t been investigated by Sue Gray”. Several MPs tackled Johnson directly about it in the Commons chamber. Justin Madders, a former shadow minister, asked: “Can he confirm for the record everyone who was there that evening and [that] there was no alcohol, no music, or anything else that people might reasonably conclude constituted a party?” Johnson declined and said he had “nothing to add” to Gray’s findings. Joanna Cherry, an SNP MP, also said she was “puzzled as to why the Abba party in the prime minister’s flat had never been investigated either by Sue Gray or the Met police”. She added: “So can I ask the PM what can be done by way of an independent investigation to assure me and my constituents that the Met police have not been nobbled?” In response, Johnson told her to “look more closely at Sue Gray’s report because I think she will find the answer she needs”. Pressed further on the flat party at a press conference on Wednesday, Johnson said Gray had pointed out the Downing Street flat had a “dual use”. He added: “Historically, prime ministers have used it for meetings. The event in question was a work meeting and the Metropolitan police did investigate it and that was certainly the outcome of their investigation.”