In his statement, Johnson also criticises former senior civil servant Sue Gray, who led the investigation into the Partygate scandal, and is soon expected to work for Labour leader Sir Keir Starmer.
Johnson says he no longer believes "that it is any coincidence" that she will soon become "chief of staff designate" of the Labour leader.
"Nor do I believe that it is any coincidence that her supposedly impartial chief counsel, Daniel Stilitz KC, turned out to be a strong Labour supporter who repeatedly tweeted personal attacks on me and the government," he says.
This is of course Johnson's view of things as he sees them. BBC News will be seeking a statement from Gray and the Labour Party.
Johnson fires scathing attack at current Tory government
In his resignation statement, Boris Johnson reaffirms his party's need to deliver on the 2019 manifesto which he was elected on and one he says "was endorsed by 14 million people".
"We need to cut business and personal taxes - and not just as pre-election gimmicks - rather than endlessly putting them up," he says in his statement.
"We must not be afraid to be a properly Conservative government.
"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?
"We should remember that more than 17 million voted for Brexit."
The majority I won is clearly at risk - Johnson
ReutersCopyright: Reuters
Boris Johnson's resignation statement is long - over 1,000 words - and we're still going through it to pull out key lines.
In it, he appears to take a parting swipe at Rishi Sunak's government.
He says: "When I left office last year the government was only a handful of points behind in the polls. That gap has now massively widened.
"Just a few years after winning the biggest majority in almost half a century, that majority is now clearly at risk. 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."
Johnson blasts 'egregious bias' of committee chair
The former prime minister's resignation triggers a by-election in his marginal west London seat
of Uxbridge and South Ruislip.
"I am very sorry to leave my wonderful constituency. It has been a huge honour to serve them, both as mayor and MP," he says in his lengthy statement.
"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."
Harman has yet to respond to Johnson's allegations. She chaired the committee's probe into Johnson after the sitting chair - Labour MP Sir Chris Bryant - recused himself because he had issued negative public statements about Johnson's conduct over Partygate.
Johnson 'bewildered and appalled' at Partygate investigation
In a lengthy statement, Johnson say he is quitting his seat because he is "bewildered and appalled" at the Privileges Committee's investigation into whether he misled MPs over Partygate.
He says he received a letter from the committee which is "making it clear, much to my amazement, that they are determined to use the proceedings against me to drive me out of Parliament".
BreakingBoris Johnson resigns as MP
Boris Johnson resigns as an MP with immediate effect after receiving the Partygate report.
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Edited by Jamie Whitehead
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Sue Gray among those criticised by Johnson
In his statement, Johnson also criticises former senior civil servant Sue Gray, who led the investigation into the Partygate scandal, and is soon expected to work for Labour leader Sir Keir Starmer.
Johnson says he no longer believes "that it is any coincidence" that she will soon become "chief of staff designate" of the Labour leader.
"Nor do I believe that it is any coincidence that her supposedly impartial chief counsel, Daniel Stilitz KC, turned out to be a strong Labour supporter who repeatedly tweeted personal attacks on me and the government," he says.
This is of course Johnson's view of things as he sees them. BBC News will be seeking a statement from Gray and the Labour Party.
Johnson fires scathing attack at current Tory government
In his resignation statement, Boris Johnson reaffirms his party's need to deliver on the 2019 manifesto which he was elected on and one he says "was endorsed by 14 million people".
"We need to cut business and personal taxes - and not just as pre-election gimmicks - rather than endlessly putting them up," he says in his statement.
"We must not be afraid to be a properly Conservative government.
"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?
"We should remember that more than 17 million voted for Brexit."
The majority I won is clearly at risk - Johnson
Boris Johnson's resignation statement is long - over 1,000 words - and we're still going through it to pull out key lines.
In it, he appears to take a parting swipe at Rishi Sunak's government.
He says: "When I left office last year the government was only a handful of points behind in the polls. That gap has now massively widened.
"Just a few years after winning the biggest majority in almost half a century, that majority is now clearly at risk. 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."
Johnson blasts 'egregious bias' of committee chair
The former prime minister's resignation triggers a by-election in his marginal west London seat of Uxbridge and South Ruislip.
"I am very sorry to leave my wonderful constituency. It has been a huge honour to serve them, both as mayor and MP," he says in his lengthy statement.
"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."
Harman has yet to respond to Johnson's allegations. She chaired the committee's probe into Johnson after the sitting chair - Labour MP Sir Chris Bryant - recused himself because he had issued negative public statements about Johnson's conduct over Partygate.
Johnson 'bewildered and appalled' at Partygate investigation
In a lengthy statement, Johnson say he is quitting his seat because he is "bewildered and appalled" at the Privileges Committee's investigation into whether he misled MPs over Partygate.
He says he received a letter from the committee which is "making it clear, much to my amazement, that they are determined to use the proceedings against me to drive me out of Parliament".
BreakingBoris Johnson resigns as MP
Boris Johnson resigns as an MP with immediate effect after receiving the Partygate report.
Stick with us for key updates.