Johnson had been under investigation by a parliamentary inquiry looking into whether he misled the House of Commons about lockdown-breaking parties in Downing Street during the COVID-19 pandemic.

After Johnson received a confidential letter from the committee, he accused lawmakers on the committee investigating him of acting like a "kangaroo court" and being determined to end his political career.

"I am being forced out by a tiny handful of people, with no evidence to back up their assertions, and without approval even of Conservative party members let alone the wider electorate," Johnson said in a statement.

Parliament's privileges committee - the main disciplinary body for lawmakers - has the power to recommend Johnson be suspended from parliament. If the suspension is for more than 10 days, voters in his constituency can demand he stand for re-election to continue as their representative.

Johnson hinted that he planned to make a return to frontline politics, declaring he was leaving parliament "for now".

But the decision to resign may be the end of his 22-year political career, where he rose from parliament to mayor of London and then built a profile that tipped the balance of the 2016 European Union referendum in favour of Brexit.

Johnson, whose premiership was cut short in part by anger in his own party and across Britain over COVID rule-breaking lockdown parties in his Downing Street office and residence, said the committee had "not produced a shred of evidence" that he deliberately or knowingly misled parliament.

"I am not alone in thinking that a witch hunt is underway to take revenge for Brexit and ultimately to reverse the 2016 referendum result," he said. "My removal is the necessary first step, and I believe there has been a concerted attempt to bring it about."

The investigation is chaired by a senior Labour Party lawmaker, but the majority of lawmakers on the committee are Conservatives.

A parliamentary spokesperson said the committee had no immediate reaction to Johnson's statement.

ATTACK ON SUNAK

The resignation will trigger a by-election for his constituency in west London. It is the second in a day for Prime Minister Rishi Sunak after an ally of Johnson, Nadine Dorries, announced she would step down.

Angela Rayner, deputy leader of the Labour Party, said: "The British public are sick to the back teeth of this never-ending Tory soap opera played out at their expense."

Johnson came to power nearly four years ago, promising to deliver Brexit and rescue it from the bitter wrangling that followed the 2016 referendum. He shrugged off concerns from some fellow Conservatives that his narcissism, failure to deal with details, and a reputation for deceit meant he was unsuitable.

Some Conservatives enthusiastically backed the former journalist and London mayor, while others, despite reservations, supported him because he was able to appeal to parts of the electorate that usually rejected their party.

That was borne out in the December 2019 election. But his administration's combative and often chaotic approach to governing and the scandals exhausted the goodwill of many of his lawmakers. Opinion polls show he is no longer popular with the public at large.

Johnson used his resignation statement to deliver an attack on the premiership of Sunak, whom he partly blames for ending his government. The men, who worked closely together during the pandemic, have had a feud since Sunak resigned as finance minister last summer in protest against Johnson's leadership.

"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.

"Our party needs urgently to recapture its sense of momentum and its belief in what this country can do."