The cross-party Privileges Committee, which is conducting the investigation into whether Mr Johnson — who was prime minister at the time — misled parliament, said evidence suggests he would have been aware restrictions were being flouted.
The committee is preparing for a live showdown with the ex-Tory leader later this month.
Mr Johnson was fined for breaking his own Covid rules during the pandemic.
He said that a mid-pandemic leaving party in No 10 was “probably the most unsocially distanced gathering in the UK right now”, according to written evidence.
WhatsApp messages given to the inquiry show advisers were “struggling” to defend how parties were held in line with the rules, with one aide conceded an excuse “blows another great gaping hole in the PM’s account”.
Mr Johnson released a statement claiming the inquiry’s interim report showed he was being “vindicated” as he raised concerns about civil service investigator Sue Gray’s move to Keir Starmer’s office.
But the committee said: “The evidence strongly suggests that breaches of guidance would have been obvious to Mr Johnson at the time he was at the gatherings.
“There is evidence that those who were advising Mr Johnson about what to say to the press and in the House were themselves struggling to contend that some gatherings were within the rules.”
The committee defended its inquiry as being “not based on Ms Gray’s report” but on evidence including witnesses, WhatsApps, emails and photographs from a Downing Street photographer.
It emerged on Thursday that Ms Gray had quit her Cabinet Office role and is poised to become chief of staff to Labour leader Mr Starmer.
Prime minister Rishi Sunak's official spokesman said the Cabinet Office is looking into the circumstances of her resignation.
Messages between Downing Street’s then-communications director Jack Doyle and a No 10 official discussing the birthday gathering held for Mr Johnson in 2020, for which he was fined by police, is among the evidence.
“I’m struggling to come up with a way this one is in the rules in my head,” Mr Doyle wrote.
In response to a suggestion that the event as “reasonably necessary for work purposes”, he said: “Not sure that one works, does it? Also blows another great gaping hole in the PM’s account doesn’t it?'”
One No 10 official in another exchange said a colleague was “worried about leaks” over Mr Johnson having attended the event.
The committee will examine what Mr Johnson knew, with him making various denials to the Commons, including telling MPs on December 8, 2021, that no rules or guidance had been broken in No 10 — though Ms Gray and the police have already concluded that was not correct.
It also pointed to Mr Johnson’s failure to tell the House of Commons about his own knowledge of the gatherings in which the rules or guidance had been broken.
The report said MPs may also have been misled by Mr Johnson’s assertion that he relied upon repeated assurances that the rules had not been broken.
The committee said: “It appears that Mr Johnson did not correct the statements that he repeatedly made and did not use the well-established procedures of the House to correct something that is wrong at the earliest opportunity.”
Mr Johnson may have broken the rules when giving the impression that there needed to be an investigation by Ms Gray.
“While repeatedly making that statement to the House, he appears to have had personal knowledge that he did not reveal,” the committee said.
He told the committee he had “no relevant material” after repeated requests for evidence and still “has not provided us with a written submission”.
In a public statement released moments after the report was published, Mr Johnson said: “I believe that their labours have helped establish the obvious truth: It is clear from this report that I have not committed any contempt of parliament.
“It is also clear that what I have been saying about this matter from the beginning has been vindicated.
“That is because there is no evidence in the report that I knowingly or recklessly misled parliament, or that I failed to update parliament in a timely manner.”
The report makes clear that the publication was “not the final conclusions”, with Mr Johnson scheduled to give oral evidence live on television in the week starting March 20.
Mr Johnson said he “relied upon advice from officials”, so when he told MPs “that the rules and the guidance had been followed, that was my honest belief”.
He said it was “surreal” and “particularly concerning” to discover the committee was partially relying on findings from Ms Gray, who is moving into a high-level Labour position.
Mr Johnson said: “I leave it to others to decide how much confidence may now be placed in her inquiry and in the reports that she produced.”