Boris Johnson is resigning as an MP after accusing a Commons investigation into whether he misled Parliament over partygate of attempting to “drive me out”, the former prime minister said in a statement.

Boris Johnson said he was quitting his seat and triggering an immediate by-election because he was “bewildered and appalled” at the Privileges Committee’s investigation into him.

He said he had received a letter from the committee which is investigating whether he lied to MPs over partygate “making it clear, much to my amazement, that they are determined to use the proceedings against me to drive me out of Parliament”.

He 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,” he said.

“I am very sorry to leave my wonderful constituency. It has been a huge honour to serve them, both as mayor and MP.”

He added: “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.”

In a rallying call to his followers, Boris Johnson used his resignation statement to deliver a stinging attack on Rishi Sunak’s Government.

“When I left office last year the government was only a handful of points behind in the polls. That gap has now massively widened,” he said.

“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.