The senior official investigating lockdown parties in Downing Street said she was able to collect only "limited" evidence about an event in the prime minister's flat.
Sue Gray's report said she stopped her inquiry into the gathering on 13 November 2020, after the Metropolitan Police started its investigations.
It was among a number of gatherings that were "necessarily limited".
At a news conference, Boris Johnson said the event was "a work meeting".
Earlier he was asked if he was at the event to conduct a "job interview".
He told MPs the matter had been "extensively investigated" and had nothing to add beyond what was in Ms Gray's report.
The gathering in November 2020 was one of 16 looked at by Ms Gray as part of her inquiry into lockdown parties in Downing Street and other government buildings during the pandemic.
The report states that it would not have been "appropriate to continue" with the investigation into any parties once the police inquiries had started, and that no further inquiries into events was undertaken after the police investigations had finished.
In newspaper reports, the gathering on 13 November has been described as a party at Mr Johnson's No 11 flat to mark the departure of his former advisers Dominic Cummings and Lee Cain.
The pair left Downing Street in acrimonious circumstances after internal battles over Mr Cummings's role as Mr Johnson's chief adviser.
The Mail on Sunday reported that the party was organised by the prime minister's wife, Carrie Johnson, and involved dancing, drinking and music by Swedish pop group Abba.
But Downing Street denied Mrs Johnson had held a party and last month, the Times reported that Mr Johnson was interviewing, Henry Newman, a close friend of his wife, about a job at No 10.
Earlier on 13 November, the prime minister was pictured drinking with staff at a leaving do for Mr Cain in the press office area of No 10.
The Met Police said Covid-19 regulations were breached on 13 November 2020, but did not specify at which event, or how many people were fined.
In her long-awaited report, released on Wednesday, Ms Gray said "a meeting was held in the No 10 flat from some time after 18:00" on 13 November, 2020.
Five special advisers attended to discuss the handling of Mr Cain's departure and that of Mr Cummings, the report says.
"The prime minister joined them at about 20.00," the report adds.
"Food and alcohol were available. The discussion carried on later into the evening with attendees leaving at various points."
Ms Gray says the "information collected on this gathering is limited" because she had only just started obtaining evidence when the Met Police launched its investigation.
"At this point I stopped my investigation, given the need to avoid any prejudice to the police investigation," Ms Gray says.
Once the Met Police finished its investigation last week, Ms Gray "concluded it was not appropriate or proportionate" to carry out further inquiries about the gathering.
As part of its investigation, the Met Police sent out questionnaires to suspected Covid rule-breakers but there were no interviews under caution.
In Parliament on Wednesday, after Mr Johnson made a statement on Ms Gray's report, Labour MP Catherine West asked about the "Abba party he attended briefly".
She said: "Can he confirm he was only in his flat and he met Henry Newman to discuss a job, and can he confirm what the other special advisers were doing - were they part of the job interview as well?"
Mr Johnson replied: "That evening was extensively investigated to the best of my knowledge and I don't believe I can improve on what Sue Gray has had to say."