In an update of its work which will be used as a basis for its questioning of the former leader, a panel of lawmakers known formally as the Privileges Committee said it had collected evidence from multiple witnesses about the parties, but that Johnson had yet to provide them with a written submission.
Despite the report being an update rather than a final conclusion, Johnson was quick to say it cleared him of the charge of committing "contempt of parliament", repeating his argument that he was not aware that any gatherings were in breach of his own coronavirus rules.
"Mr Johnson has accepted the Committee's invitation to give oral evidence in public in the week beginning 20 March," the Committee of Privileges said in a statement.
The so-called partygate scandal ultimately led to the downfall of Johnson, after months of reports that he, alongside other senior government figures, had been present at parties when most of the rest of Britain were forced to stay at home.
The outcry prompted the resignations at that time of much of his top team of government ministers, including the current Prime Minister Rishi Sunak, and Johnson stepped down in September. He argues he was not aware that any of the events taking place at Downing Street broke COVID-19 rules.
At the centre of the inquiry are statements Johnson made to parliament in December 2021 about parties at Downing Street when coronavirus laws kept people at home and banned indoor gatherings.
"All guidance was followed in No. 10," Johnson told parliament that month, followed seven days later by: "I have been repeatedly assured since these allegations emerged that there was no party and that no COVID rules were broken."
The committee said the evidence "strongly suggests that breaches of guidance would have been obvious" to Johnson at the time he was at the gatherings. There is evidence that those who were advising Johnson were concerned he was breaking the rules, it added.
The committee only has the power to report its findings to parliament on whether Johnson should be found to have committed a "contempt", for which it can recommend sanctions ranging from verbal apologies to expulsion from parliament.
Its findings, and any sanction on Johnson, would have to be approved by parliament.
Johnson was defiant after the interim report.
"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," he said in a statement.
He said he relied on the guidance of officials and that there was no evidence he was advised that any event "would be against the rules or the guidance before it went ahead".
"So, when I told the House (of Commons or parliament) that the rules and the guidance had been followed, that was my honest belief," he said, casting doubt over the testimony of a senior civil servant who led another inquiry into partygate.
He said it was "surreal" that the committee might rely on Sue Gray, who has been offered a senior role in the opposition Labour Party, a move which prompted some in the governing Conservative Party to question whether she was objective in her original partygate probe.
"I leave it to others to decide how much confidence may now be placed in her inquiry and in the reports that she produced," he said.