- The cross-party committee of MPs - the majority of which are Conservative - added it would conclude its inquiry on Monday and "publish its report promptly".
- Mr Johnson's resignation now triggers a by-election in his marginal constituency of Uxbridge and South Ruislip.
Boris Johnson has stepped down as a Tory MP after claiming he was "forced out of Parliament" over Partygate.
The ex-PM saw in advance a report by the Commons Privileges Committee investigating if he misled the Commons over Downing Street lockdown parties.
In an explosive and lengthy statement, he called the committee a "kangaroo court" whose purpose "has been to find me guilty, regardless of the facts".
The committee said it had "followed the procedures and the mandate".
The cross-party committee of MPs - the majority of which are Conservative - added it would conclude its inquiry on Monday and "publish its report promptly".
Mr Johnson's resignation now triggers a by-election in his marginal constituency of Uxbridge and South Ruislip.
Delivering his announcement late on Friday evening, Mr Johnson said the draft report he had seen was "riddled with inaccuracies and reeks of prejudice", adding it was clear the committee was "determined to use the proceedings against me to drive me out of Parliament".
"They have still not produced a shred of evidence that I knowingly or recklessly misled the Commons," he said, insisting "I did not lie".
He also accused its chairwoman, Labour's Harriet Harman, of "egregious bias", saying he was "bewildered and appalled" at how he was being forced out.
The ex-prime minister previously admitted misleading Parliament when he gave evidence to the committee in a combative hearing in March - but denied doing it on purpose.
He said social distancing had not been "perfect" at gatherings in Downing Street during Covid lockdowns but insisted the guidelines, as he understood them, were followed at all times.
Mr Johnson also used his letter to attack the direction of the government, saying "we must not be afraid to be properly Conservative" and warning the party's majority was at risk.
"We need to show how we are making the most of Brexit and we need in the next months to be setting out a pro-growth and pro-investment agenda," Mr Johnson argued.
"Why have we so passively abandoned the prospect of a Free Trade Deal with the US? Why have we junked measures to help people into housing or to scrap EU directives or to promote animal welfare?"
It was a direct aim at Prime Minister Rishi Sunak - hours after he stepped off a plane from Washington where Mr Sunak was not talking about a free trade agreement with the US.
And Mr Johnson's statement was an attempt to rally Brexiteers in his party, suggesting his demise was driven by a motivation to "reverse the 2016 referendum result".
The statement contained further criticism of former senior civil servant Sue Gray, who investigated lockdown gatherings at Number 10.
"I am afraid I no longer believe that it is any coincidence" that she will soon become "chief of staff designate" of the Labour leader Sir Kier Starmer, Mr Johnson wrote.
Ending his 1,000-word letter, Mr Johnson said he was "very sad to be leaving Parliament" before adding - "at least for now" - for anyone thinking he is about to retreat into obscurity.
Mr Johnson's exit will trigger a by-election in his west London seat, which he held with a 7,000 vote majority in the 2019 election.
The Conservatives will also have to defend the Mid Bedfordshire seat of Nadine Dorries - a close ally of Mr Johnson - after she stepped down as an MP earlier on Friday.
Mr Johnson's dramatic move came after he was given the committee's findings, including details of criticisms it intended to make and evidence to support its conclusion.
He had faced a potential by-election if MPs recommended a suspension from the Commons as a punishment for misleading Parliament.
Responding to his statement, a Privileges Committee spokesperson said: "The committee has followed the procedures and the mandate of the House at all times and will continue to do so.
"Mr Johnson has departed from the processes of the House and has impugned the integrity of the House by his statement. The committee will meet on Monday to conclude the inquiry and to publish its report promptly."
Elsewhere, Labour deputy leader Angela Rayner condemned what she called a "never-ending Tory soap opera".
For the Liberal Democrats, deputy leader Daisy Cooper said: "Good riddance."
And SNP deputy Westminster leader Mhairi Black said Mr Johnson "jumped before he was pushed", adding "no-one in Scotland will be sorry to see the back of him".
However, former home secretary Priti Patel, who was made a Dame in his resignations honours list also announced on Friday, praised Mr Johnson for his work as prime minister on the issues of Ukraine and Brexit, describing him as "a political titan".
Boris Johnson's local Conservative association chairman, Richard Mills, said the former PM "has delivered on his promises to local residents".
Another sitting MP announced in the resignation honours list, Sir Michael Fabricant, criticised the Privileges Committee for what he called its "disgraceful treatment" of the former prime minister.
Mr Johnson was prime minister from July 2019 until September 2022, and has been an MP since 2001 - although not continuously, having served as mayor of London between 2008 and 2016.