Boris Johnson will be found to have committed “multiple” contempts of parliament in a report by MPs into his conduct as prime minister during the partygate scandal, according to people close to the inquiry.

The House of Commons privileges committee is set to publish its long-awaited report about Johnson on Thursday morning following a 14-month investigation.

The report concluded Johnson had made “a number of contempts” of parliament, including being found to have misled MPs in statements he made in the Commons about Downing Street parties held during coronavirus lockdowns, said two people close to the committee.

While the committee’s primary focus will be on Johnson, it will also censure the behaviour of MPs who have criticised its probe but will stop short of naming them, added these people.

The committee has been probing whether Johnson deliberately misled parliament after stating while prime minister that Covid-19 rules were followed at all times following media reports of Number 10 gatherings held during pandemic restrictions.

Johnson announced on Friday he was immediately stepping down as Conservative MP for Uxbridge and South Ruislip — after accusing the cross-party committee of behaving like a “kangaroo court” engaged in a “political hit job” against him.

While the committee has powers to recommend sanctions against MPs, including suspension from the Commons that can lead to a by-election, Johnson’s decision to quit pre-empted such an outcome.

One of the people close to the committee said Johnson’s decision to disclose some of the panel’s draft conclusions last Friday had been found to be a contempt.

This person added: “There is reference [in the report] to other issues beyond simply the contempt of parliament in what [Johnson] said at the [Commons] despatch box [about Downing Street parties].

“Disclosing the outcome of the inquiry is itself a contempt of parliament. It is clear cut, it does not require further investigation.”

This finding by the panel was anticipated by others with knowledge of its workings.

Dominic Grieve, a former member of the Commons privileges committee and ex-Tory attorney-general, said Johnson would have been given the draft report in confidence before speaking out about its contents.

“If he breached that confidence, then he has acted in contempt of the committee,” he added. “I don’t think there’s a grey area.”

Johnson said in statement: “I am not guilty of any contempt of parliament and did not knowingly or wittingly mislead the Commons.”

The committee’s report is also set to raise concerns about MPs who have ridiculed the panel but is not expected to identify them by name.

Conservative MPs Sir Jacob Rees-Mogg and Dame Andrea Jenkyns, both close allies of Johnson, have branded the committee’s inquiry a “kangaroo court”.

One of the people close to the committee said: “Parliament regulates its own affairs. The courts are not involved. If it can’t do so in a reasonable manner, there is a question mark about the future independence of the privileges system.”

Johnson on Wednesday called on a member of the privileges committee to resign over reports the MP had attended a gathering in parliament during Covid restrictions.

Tory MP Sir Bernard Jenkin attended a drinks event in parliament in December 2020 when London was in so-called Tier 2 restrictions and gatherings of more than six people were not allowed indoors, according to the Guido Fawkes website.

Johnson said it was “outrageous and a total contempt of parliament” if true, adding that Jenkin “has no choice but to explain his actions to his own committee, for his colleagues to investigate and then to resign”.

Jenkin has been approached for comment.

Other MPs accused Johnson of seizing on the report about Jenkin in an attempt to deflect attention away from his own conduct.

Former Tory cabinet minister David Davis said: “The difficulty for Boris is, if you’re in a court case, it’s not a viable excuse to say ‘One of the jury is imperfect’, even if it’s true and it may not be . . . It’s a distraction from the real issue — that he is guilty of misleading the House.”

Daisy Cooper, deputy leader of the Liberal Democrats, said: “This a typical distraction tactic from Boris Johnson that doesn’t change the fact he broke the law and lied about it.”