Boris Johnson deliberately misled British lawmakers over lockdown-breaking parties in Downing Street during the coronavirus pandemic, a powerful committee concluded on Thursday, releasing publicly the findings that prompted Mr. Johnson’s angry resignation from Parliament last week.

The 108-page document, produced by the House of Commons privileges committee, offered a damning verdict on Mr. Johnson’s honesty and integrity, concluding that his conduct in misleading lawmakers who questioned him, including about his publicly documented violations of lockdown policy, was deliberate and that he had committed “a serious contempt” of the House.

“The contempt was all the more serious because it was committed by the prime minister, the most senior member of the government,” the report concluded. “There is no precedent for a prime minister having been found to have deliberately misled the House.” In addition, it said, “He misled the House on an issue of the greatest importance to the House and to the public, and did so repeatedly.”

Had he remained a lawmaker, the punishment recommended by the committee would have been a 90-day ban from Parliament, one of the most severe options it could have suggested. The report also recommended that the former prime minister’s parliamentary pass should be revoked, preventing him from visiting Parliament as he would normally be entitled to do.

The brutal verdict on the character of the former prime minister raises questions about whether Mr. Johnson has any prospects of reviving his political career and of returning to Parliament, something that he has hinted he would like to do.

Mr. Johnson was sent a draft of the report last week and promptly resigned from the House of Commons, characterizing the committee investigating him as a “kangaroo court” bent on a politically motivated witch hunt against him. In fact, most of its members are from the Conservative Party, which Mr. Johnson led until last year, and two are prominent supporters of Brexit, his flagship policy.

The privileges committee, which polices some internal parliamentary matters, had the power to recommend a suspension from Parliament that might have forced Mr. Johnson into an election to retain his seat. Confronted by that uncertainty-laden prospect, Mr. Johnson quit rather than risk his track record as an election winner.

But in denouncing the committee, Mr. Johnson only hardened its judgment. Its members have been offered added security following comments questioning their impartiality made by the former prime minister and his supporters.

In light of Mr. Johnson’s reaction, the committee justified the penalties it recommended by saying that he had been “complicit in the campaign of abuse and attempted intimidation of the Committee,” as well as impugning its work “and thereby undermining the democratic process of the House.”

Mr. Johnson’s explosive resignation statement last week coincided with a separate dispute over the honors that he planned to bestow on supporters, igniting a public confrontation with Rishi Sunak, who succeeded Liz Truss as prime minister after her brief, ill-fated, time in Downing Street last year.

The document released on Thursday examined in detail the veracity of Mr. Johnson’s account of how he and his senior aides behaved during the pandemic. Even as rumors circulated of parties and social mixing in violation of the rules, Mr. Johnson told Parliament that he had received assurances that all lockdown rules were complied with in Downing Street.

Yet ultimately Mr. Johnson became the first sitting prime minister to be fined by the police for breaking the law. More revelations emerged, and the “partygate” scandal became one of several that contributed to his resignation under pressure as prime minister last year.

The issue at stake for the committee was not the rule-breaking, but the way Mr. Johnson had denied it repeatedly to lawmakers.

Unsurprisingly, given his previous statements, Mr. Johnson on Thursday rejected the damning verdict on his behavior.

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

When Mr. Johnson appeared before the committee in March, he acknowledged making misleading statements in Parliament when he assured lawmakers earlier that there was no breach of lockdown rules. But he denied knowingly making misstatements. “I am here to say to you, hand on heart, that I did not lie to the House,” he said at the time. “When those statements were made, they were made in good faith on the basis of what I honestly knew and believed at the time.”

Yet Mr. Johnson accepted that he could not recall being given specific reassurances by any of his most senior civil servants that lockdown rules and guidance had been observed at all times in Downing Street.

Instead, he cited advice from two political aides, prompting the committee chair, Harriet Harman, to ask Mr. Johnson whether he had relied on “flimsy” reassurances.

In its report on Thursday, the committee drew attention to what it called Mr. Johnson’s “after-the-event rationalizations” of the nature and extent of the assurances he had received from his principal private secretary and “others whose advice would have been authoritative” regarding whether the gatherings in question were allowed under the government’s pandemic rules.

It added that Mr. Johnson had tried to “rewrite the meaning” of the Covid-19 rules and guidance in effect at the time “to fit his own evidence.” That included his assertion that “a leaving gathering or a gathering to boost morale was a lawful reason to hold a gathering.”

When he appeared before the committee, the former prime minister rejected a charge that he had been reckless in his statements. In doing so, he perhaps closed off one potential route for the committee to recommend a lesser punishment that might have allowed him to stay in Parliament without the risk of an election.

Since his ouster from Downing Street last year, Mr. Johnson has made little secret of wanting his old job as prime minister back and, on announcing his resignation from Parliament on Friday, he added the proviso that he was quitting the House of Commons “for now.” Without a seat, a political comeback — which anyway looks unlikely — would be impossible.

The former prime minister’s career has seen many dizzying high and dramatic lows. So renowned is he for escaping his difficulties that another former prime minister, David Cameron, once described Mr. Johnson as the “greased piglet” of British politics.

But Mr. Johnson’s latest setback suggested the limits of his support among Conservative lawmakers, with relatively few of them rallying to his defense.