Boris Johnson has been accused of being a coward for quitting as an MP before the findings of a cross-party investigation into whether he lied to the Commons has been published.

Angela Rayner, Labour’s deputy leader, said the former prime minister had “jumped” to avoid facing a potential by-election in his marginal constituency.

Mr Johnson dramatically resigned as MP for Uxbridge and South Ruislip on Friday and took aim at the Commons Privileges Committee in a scathing 1,000-word statement.

He said the seven-person panel, which is chaired by veteran Labour MP Harriet Harman but has a Conservative majority, was on a “witch hunt” and argued it was attempting to use its investigation to “drive me out of Parliament”.

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.

At the time when the public needed him the most, he basically was partying and lying to them at a time when they couldn’t see their loved ones

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

Ms Rayner said it was “tosh” for the former Tory leader to suggest the partygate probe was unfair and accused him of running away from a potential by-election test.

“To me, he is a coward,” the senior Opposition MP told BBC Radio 5 Live.

“He knows that the Privileges Committee has seen through this fiasco and he has jumped.

“He could have defended himself, he could have gone to his constituents and fought the suspension, and he has decided he is not going to do that because he knows he is in the wrong.”

She said Mr Johnson had “truly in the most devastating way” let voters down who had “put their trust” in him at the 2019 general election when he secured a landslide victory.

“At the time when the public needed him the most, he basically was partying and lying to them at a time when they couldn’t see their loved ones,” she told BBC Radio 4’s Today programme.

“And that is unforgivable.”

Will Walden, who previously worked as a spokesman for Mr Johnson, said his former boss had “seen the writing on the wall” that he could be ousted in a potential by-election triggered by the Privileges Committee’s sanction.

He told Today: “By going, as he has, all guns blazing, he is able to avoid defeat, he is able to blame pretty much everyone else, including it seems anyone that voted Remain in 2016.

“But it is so Boris. He told the committee that if they found against him, he wouldn’t respect the outcome – and so it has proved, there is no great surprise here.”

Sir Chris Bryant, the Labour chairman of the Privileges Committee who recused himself from the investigation into Mr Johnson, raised the possibility that further contempt of Parliament charges could be levelled against the former British leader after his “narcissistic rant”.

Ms Harman’s 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, 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”.

Speaking to the Today programme, Sir Chris said: “They may want to conclude that there has been an additional contempt of Parliament by the way that Boris Johnson has behaved in the last 24 hours and in the attacks on the committee, which are in effect an attack on the whole House.

“I don’t think anybody can now be in any doubt that Boris Johnson holds Parliament in contempt.”

Mr Johnson’s resignation means Prime Minister Rishi Sunak faces two difficult by-elections, with former culture secretary Nadine Dorries also announcing on Friday that she was departing the Commons immediately, rather than waiting until the next election.

Uxbridge and South Ruislip was in Labour’s top 100 target seats at the next election even before Mr Johnson’s resignation and the Liberal Democrats have briefed that they could potentially pull-off an upset in Ms Dorries’ former Mid Bedfordshire constituency.

To win in Mid Bedfordshire, where Labour came second in 2019, the Lib Dems would need a swing of 23% — less than the swings they achieved in by-elections held in the likes of Chesham and Amersham, and Tiverton and Honiton in recent years.

Meanwhile, Liberal Democrat leader Sir Ed Davey has ruled out a pact with Labour to encourage tactical voting to win the two by-elections.

“There’ll be no pacts, no deals. We will fight both by-elections. Voters will make the decision,” he told the Today programme.

Mr Sunak, who Mr Johnson in his resignation statement suggested was failing to deliver on his 2019 election manifesto, has yet to comment on his predecessor’s Westminster departure.

Mr Johnson’s Commons exit came only hours after his resignation honours list was published in which he gave peerages and other awards to close allies.