Tory MPs fear a cover up over the 'Abba party' in the Downing Street flat after Sue Gray admitted she did not fully investigate the gathering.
Her long-awaited report into the Partygate scandal admitted her information on the event was "limited" as she had stopped when the police probe began.
After Scotland Yard wrapped up its inquiry last week she decided it was "not appropriate or proportionate" to carry on looking at the event.
Boris Johnson's wife Carrie is alleged to have blasted out the Abba song 'Winner Takes It All' at the event in the flat on 13 November 2020.
Sources told the Mirror last year that attendees were "all getting absolutely plastered" to celebrate the departure of bitter rival Dominic Cummings.
The claims have previously been denied by Mrs Johnson's spokeswoman.
But Tory MPs questioned why Ms Gray had not fully investigated the gathering with one suggesting "it looks like a cover-up".
Another said: "It took place in his flat and therefore was potentially the most damaging so I just can't understand why she didn't look into it properly".
Labour MP Chris Bryant, chair of the Commons standards committee, said he was “mystified" why the event had not been probed further.
The SNP's Joanna Cherry also said she was “puzzled" by the failure to investigate and asked the PM to assure her the Met Police "have not been nobbled?"
Mr Johnson told her to “look more closely" at the report because "I think she will find the answer she needs”.
At his Downing Street press conference, the PM told the Mirror that Ms Gray had previously pointed out the official flat had "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.”
Ms Gray confirmed that Mrs Johnson and five special advisors were in the flat following the departure of the PM's former top aide.
She said the PM joined them a couple of hours later and "food and alcohol" were available at the event which carried on later into the evening.
In her report, Ms Gray said: "The information collected on this gathering is limited as the process of obtaining evidence had only just been commenced when the Metropolitan Police announced their own investigations, which included events on the 13 November 2020.
"At this point I stopped my investigation, given the need to avoid any prejudice to the police investigation.
"Following the Metropolitan Police announcement on 19 May 2022 I considered whether or not to conduct any further investigation into this event but concluded it was not appropriate or proportionate to do so."
The couple escaped fines for the gathering, which was investigated by Scotland Yard, although it was unclear whether this was the case for other attendees.
Mrs Johnson, who took legal advice before submitting her questionnaire, is said to have told police the gathering was a work event.
The PM is believed to have claimed he was interviewing one of his wife's close friends, Henry Newman, for a job in another part of the official residence.
One insider said: "Why did he have to interview him in the flat when he could've done it downstairs? It doesn't make any sense."
