Keir Starmer, the leader of Britain s Labour Party makes his speech at the party s annual conference in Liverpool, England, Tuesday, Sept. 27, 2022. AP
Labour leader Starmer tweeted that Prime Minister Rishi Sunak "must finally find a backbone, call an election, and let the public have their say on 13 years of Tory failure.
"This farce must stop. People have had enough of a shambolic Tory government and a weak prime minister no one voted for."
Johnson announced on Friday he was leaving as a member of parliament, claiming he had been forced out in a stitch-up by his political opponents.
The 58-year-old Johnson has been under investigation by a cross-party committee about whether he deliberately lied to parliament over parties when he was in office.
As the committee prepares to make public its findings, Johnson said they had contacted him "making it clear... they are determined to use the proceedings against me to drive me out of parliament".
By quitting, Johnson avoids the consequences of a humiliating fight to remain an MP in his Uxbridge and South Ruislip constituency in northwest London where he holds a slim majority of just over 7,000.
He denounced the committee, chaired by veteran opposition Labour MP Harriet Harman, as a "kangaroo court".
"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... with such egregious bias," he said.
Nadine Dorries, one of Johnson's allies quit on Friday, while another, Nigel Adams, resigned on Saturday, triggering three by-elections for a government languishing in the polls.
Speculation is rife about Johnson's next move, and whether he will attempt to run to become an MP again at the next general election, due next year.
Influential Johnson supporter Jacob Rees-Mogg wrote in the Mail on Sunday that said the former Tory leader could "easily get back into parliament at the next election."
He also warned party officials against blocking such a bid, saying it would "shatter our fragile party unity and plunge the Conservatives into civil war."
Sunak was finance minister under Johnson, and it was his resignation that ultimately triggered his then boss's demise.
The party is still divided as a result, but had recently arrived at an uneasy truce that now looks to be under serious threat.