Boris Johnson has been branded a “coward” and a “disgrace” for quitting as an MP before the findings of a parliament investigation into whether he lied to MPs about partygate were published.

Members of the opposition lined up to criticise the former prime minister, who took aim at the Commons privileges committee in a blistering 1,000-word statement as he resigned on Friday.

Mr Johnson claimed the cross-party group, which has a Tory majority, were “determined to find him guilty” of misleading parliament and claimed a “witch hunt” was under way to take revenge for Brexit.

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But Angela Rayner, Labour’s deputy leader, said the former prime minister had “jumped” to avoid facing a potentially humiliating by-election in his marginal Uxbridge and South Ruislip constituency.

“I think Boris Johnson has shown himself to be a coward once more,” she told Sky News.

“He’s a man that can never hold his hands up to what he’s done. And I think he’s an absolute disgrace.”

Ms Rayner pointed out that the Commons get to vote on any recommended suspension from the privileges committee and, if 10 days or more is agreed, Mr Johnson’s constituents would then get a say if there should by a by-election.

She said Mr Johnson had chosen to “dodge all of that because he knows he’s not going get through that process, because it is clear he misled parliament”.

“He’s had a fair hearing. He’s chose to run off and cower away from responsibility of what he’s done,” she added.

“Nobody made him party during lockdown. He knows full well what he should and shouldn’t have been doing. And he let the public down in the most disgraceful way.”

The committee has been investigating whether Mr Johnson lied to the Commons when he said that COVID rules were followed in Downing Street following reports that lockdown-busting parties were held during the pandemic.

It was reportedly preparing to recommend a 10-day suspension from the Commons, a conclusion which, if MPs voted for it, would have resulted in a recall petition among his constituents and a potential by-election in his west London constituency if more than 10% supported one.

Will Walden, a former spokesman for Boris Johnson, said his old boss had “seen the writing on the wall” that he could be ousted and called his exit “very Trumpian”.

“Boris hates the comparisons with Trump, but it is the language of vendetta,” he said of Mr Johnson’s statement. “It’s a long rant, and frankly, it’s deeply misleading in places. But it’s very Boris.”

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Chris Bryant, the Labour chair of the privileges committee who recused himself from the Johnson investigation, said the former prime minister could be levelled with a new contempt of parliament charge after his “narcissistic rant”.

“He’s been so cowardly that he’s not prepared to face the music in the House of Commons,” he added.

The privileges inquiry is due to meet on Monday to finalise its conclusions and is expected to publish its report “promptly”.

In a statement released by the committee on Friday night, a spokesman said Mr Johnson had “impugned the integrity” of the Commons with his attack.

Mr Johnson said he was “bewildered and appalled” at being “forced out, anti-democratically” by a probe that he claimed had set out from the beginning to “find me guilty, regardless of the facts”.

His resignation means Rishi Sunak now faces the prospect of two by-elections, with Nadine Dorries – one of Mr Johnson’s closest allies – also announcing on Friday that she was vacating her seat effective immediately.

The two main opposition parties have vowed to fight hard to win the seats – but the Lib Dems insisted on Saturday there would be no pact with Labour to oust the Tories.

Lib Dem leader Sir Ed Davey said: “There’ll be no pacts, no deals. We will fight both by-elections. Voters will make the decision.”