© Finnbarr WebsterMr Johnson left the door open to a possible return to the Commons.
Boris Johnson stood down as an MP on Friday night claiming a Commons inquiry was determined to find him guilty over partygate.
In a move that stunned Westminster, he accused the privileges committee of acting as a "kangaroo court" that was intent on forcing him out.
He also said there had been a "witch hunt" among some Tory MPs who saw his removal as a necessary first step to the reversal of Brexit.
The former prime minister also attacked Rishi Sunak, accusing him of abandoning key Conservative positions as he called on the Tories to recover their "mojo".
Mr Johnson's decision has triggered a by-election and means he has walked away from front line politics with uncertainty about what comes next.
However, he left the door open for a return to the Commons, saying he was leaving Parliament "for now".
In a statement, Mr Johnson said the privileges 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.
He added: "Their purpose from the beginning has been to find me guilty, regardless of the facts. This is the very definition of a kangaroo court."
The privileges committee wrote a "warning letter" to Mr Johnson earlier this week outlining how it intended to criticise him in its final report.
It is understood the committee recommended a suspension of more than 10 days, meaning a recall petition triggering a by-election, would have been possible.
The committee has been investigating whether he misled Parliament when he told MPs that there had been no lockdown parties in Downing Street.
Mr Johnson's move will create a difficult by-election for the Tories in the marginal seat of Uxbridge and South Ruislip.
His announcement, at 8pm, came just hours after Nadine Dorries, the former culture secretary, resigned and triggered a by-election in Mid Bedfordshire after a peerage she thought she was getting was blocked.
Mr Johnson's resignation honours list, in which he elevated seven of his friends and close political allies to the House of Lords, was published just a few hours before he announced his resignation.
His shock departure means one of the Conservative Party's biggest figures in the last two decades is exiting front-line politics.
In an attack on the direction in which Mr Sunak has taken the party, Mr Johnson said: "our party needs urgently to recapture its sense of momentum and its belief in what this country can do.
"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. We need to cut business and personal taxes - and not just as pre-election gimmicks - rather than endlessly putting them up.
"We must not be afraid to be a properly Conservative government."
He accused Mr Sunak of having "passively abandoned the prospect of a Free Trade Deal with the US" and asked "Why have we junked measures to help people into housing or to scrap EU directives or to promote animal welfare?
"We need to deliver on the 2019 manifesto, which was endorsed by 14 million people. We should remember that more than 17 million voted for Brexit," he said.
He also accused Harriet Harman, the chairman of the privileges committee, of having " already expressed deeply prejudicial remarks about my guilt before they had even seen the evidence", saying she should have recused herself.
He said he was naïve to believe that the proceedings could be fair, adding it was the same faith in impartiality which led him to commission Sue Gray to look into his conduct.
"It is clear that my faith has been misplaced. Of course, it suits the Labour Party, the Liberal Democrats, and the SNP to do whatever they can to remove me from parliament," he added.
"Sadly, as we saw in July last year, there are currently some Tory MPs who share that view. I am not alone in thinking that there is a witch hunt under way, to take revenge for Brexit and ultimately to reverse the 2016 referendum result.
"My removal is the necessary first step, and I believe there has been a concerted attempt to bring it about. I am afraid I no longer believe that it is any coincidence that Sue Gray - who investigated gatherings in Number 10 - is now the chief of staff designate of the Labour leader."
Mr Johnson was first elected to the House of Commons as MP for Henley in 2001 alongside David Cameron and George Osborne, as part of a new Tory generation.
He would later go on to serve two terms as London mayor - winning in the country's capital city which traditionally votes Labour - before returning to Parliament in his Uxbridge and South Ruislip seat at the 2015 general election.
On Friday, Priti Patel, the former home secretary, told the Telegraph: "Boris Johnson has served our country and his constituency with distinction.
"He led the world in supporting Ukraine and defending our values, he got Brexit done, and he secured successes for the Conservative Party not seen since Margaret Thatcher.
"Boris is a political titan whose legacy will stand the test of time."
Mr Johnson has been dubbed one of the most consequential prime ministers of recent decades due to his central role in delivering the UK's exit from the European Union.
He was the figurehead of the Leave campaign in the 2016 referendum where the country voted for Brexit and then secured the departure when in Downing Street on January 31 2020.
Mr Johnson's three-year run in Downing Street - he took office in July 2019 and departed in September 2022 - was brought to an end after a spate of ministerial resignations.
The news is complicated by the fact that Mr Johnson had long been rumoured to be considering a switch of constituencies, given he was at risk from defeat due to a slim majority in Uxbridge.
Although his statement suggests he could return to parliament, how and when such a return could come is unclear. A general election is due to be carried out before the end of next year, leaving little time for an about turn.
Any attempt to be selected as the Tory candidate in a different seat could be complicated by his tense relationship with Rishi Sunak, the Prime Minister who quit Mr Johnson's Cabinet, triggering his fall.
Revealing his resignation, Mr Johnson said: "I have today written to my Association in Uxbridge and South Ruislip to say that I am stepping down forthwith and triggering an immediate by-election.
"It is very sad to be leaving parliament - at least for now - but above all I am bewildered and appalled that I can be forced out, anti-democratically, by a committee chaired and managed, by Harriet Harman, with such egregious bias."
He added: "I did not lie and I believe that in their hearts the committee know it.
"But they have wilfully chosen to ignore the truth because from the outset their purpose has not been to discover the truth, or genuinely to understand what was in my mind when I spoke in the Commons."
Mr Johnson insisted he was innocent over partygate, saying: "I have received a letter from the Privileges Committee making it clear - much to my amazement - that they are 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.
"They know perfectly well that when I spoke in the Commons I was saying what I believed sincerely to be true and what I had been briefed to say, like any other minister. They know that I corrected the record as soon as possible; and they know that I and every other senior official and minister - including the current Prime Minister and then occupant of the same building, Rishi Sunak - believed that we were working lawfully together."
He added: "I am now being forced out of parliament by a tiny handful of people, with no evidence to back up their assertions, and without the approval even of Conservative Party members let alone the wider electorate.
"I believe that a dangerous and unsettling precedent is being set."
'After 13 years of Conservative chaos, enough is enough'
Angela Rayner, Labour's deputy leader and shadow chancellor of the duchy of Lancaster, said: "The British public are sick to the back teeth of this neverending Tory soap opera played out at their expense.
"After 13 years of Conservative chaos, enough is enough. It's time to turn the page with a fresh start for Britain with a Labour government focused on the people's priorities of tackling the cost of living crisis and building a better future."
The report into whether Mr Johnson misled MPs over partygate is now due to be published next week, with the timing brought forward after his statement.
A privileges committee spokesman 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."
Could have come out of Julian Assange’s mouth that.
He delivered on Brexit did he? Is that why we are getting boat loads of immigrants to be put in 4 star hotels still, by the day.
How long is it until one of them oppressed immigrants decides to stab up a nursery?
Brexit. Was. Never. Happening. Ever.
After Brexit and the Trump result, is it any wonder why the election/referendum process has become so in your face fraudulent?