LONDON — A committee of lawmakers rebuked former British Prime Minister Johnson on Thursday, saying he lied to Parliament about lockdown-flouting parties and was complicit in a campaign to intimidate those investigating his conduct.

The House of Commons Privileges Committee found Mr. Johnson’s actions were such a flagrant violation of the rules that they warranted a 90-day suspension from Parliament. 

That sanction would have been more than enough to trigger a by-election that could have cost Mr. Johnson his seat in Parliament, but the former prime minister avoided that ignominy by resigning on Friday after the committee gave him advance notice of its findings.

Release of the committee’s scathing 77-page report touched off an angry exchange of recriminations, with Mr. Johnson repeating his claims that the panel was a “kangaroo court” bent on ousting him from Parliament and the committee saying his defense was an after-the-fact justification that was “no more than an artifice.”

The report is just the latest episode in the “partygate” scandal that has angered the public and distracted lawmakers since local news organizations first revealed that members of Mr. Johnson’s staff held a series of parties in 2020 and 2021 when such gatherings were prohibited by pandemic restrictions.

Mr. Johnson initially denied that any parties took place, then repeatedly assured lawmakers that rules and guidance were followed at all times.

The committee, which took testimony from Mr. Johnson and senior members of his government during its 14-month investigation, concluded that those assurances were misleading and that Mr. Johnson failed to correct the record when asked to do so. This amounted to a “serious contempt” of Parliament, the panel found.

“The contempt was all the more serious because it was committed by the Prime Minister, the most senior member of the government,” the committee said. 

“There is no precedent for a Prime Minister having been found to have deliberately misled the House,” the report added. “He misled the House on an issue of the greatest importance to the House and to the public, and did so repeatedly.”

The committee also said Mr. Johnson should not be granted a pass to Parliament’s grounds.

Mr. Johnson, 58, fought back in a statement tinged by fury. He insisted he had done nothing wrong.

“The committee now says that I deliberately misled the House, and at the moment I spoke I was consciously concealing from the House my knowledge of illicit events,” Mr. Johnson said. 

“This is rubbish. It is a lie. In order to reach this deranged conclusion, the Committee is obliged to say a series of things that are patently absurd, or contradicted by the facts.”

Mr. Johnson stepped down as a lawmaker on Friday after the committee informed him in advance that he would be sanctioned. In his statement Thursday, he lashed out at the committee, saying they used their prerogatives to bring about what “is intended to be the final knife-thrust in a protracted political assassination.”

Mr. Johnson’s move to quit Parliament means he can no longer be suspended, and his seat of Uxbridge and South Ruislip will be contested in a special election in July.

Mr. Johnson and his wife, Carrie, were fined by the Metropolitan Police last year for breaching Covid laws at a birthday party for Mr. Johnson in June 2020 in his Downing Street residence and office.

Prime Minister Sunak was also among dozens of people issued with fixed-penalty notices for a series of office parties and “wine time Fridays” in 2020 and 2021 across government buildings.

Revelations of the booze-fueled gatherings, which took place at a time when millions were prohibited from seeing loved ones or even attending family funerals, angered many Britons and added to a string of ethics scandals that spelled Mr. Johnson’s downfall. 

Mr. Johnson resigned as prime minister in July 2022 after a mass exodus of government officials protesting his leadership.

Mr. Johnson has acknowledged misleading lawmakers when he assured them that no rules had been broken, but he insisted he didn’t do so deliberately.

In March he told the committee he “honestly believed” the five gatherings he attended, including a send-off for a staffer and his own surprise birthday party, were “lawful work gatherings” intended to boost morale among overworked staff members coping with a deadly pandemic.

Families of people who died in the pandemic flatly disagreed. The Covid-19 Bereaved Families for Justice UK group said the committee’s report was a painful reminder that while they were saying goodbye to loved ones on Zoom, the prime minister was holding parties.

A spokesman for the group, David Garfinkel, said Mr. Johnson should be barred from holding office again.

“Johnson has shown no remorse,” Mr. Garfinkel said in a statement.