Labour's deputy leader Angela Rayner says the main opposition party aims to trigger a vote of no confidence in parliament if the Tories do not get rid of Johnson immediately.
Britain's Labour party has threatened a bid to force Prime Minister Boris Johnson out of Downing Street immediately, following his resignation in the face of a cabinet uprising.
Johnson quit as leader of the ruling Conservative party on Thursday, after a frenzy of nearly 60 resignations in less than 48 hours in opposition to his scandal-hit reign.
But the 58-year-old, whose three-year premiership has been defined by Britain's departure from the European Union and Covid, said he would stay on until his successor is found.
Former finance minister Rishi Sunak, whose resignation on Tuesday was instrumental in Johnson's demise, launched his bid for the top job on Friday.
"Let's restore trust, rebuild the economy and reunite the country," the multimillionaire said, presenting a slick video on social media at the start of what could be a months-long campaign.
Defence minister Ben Wallace, who could also expected to declare his bid, and Sunak are among the early frontrunners, a YouGov poll of Tory members suggested.
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Meanwhile, calls are mounting for Johnson to leave straight away and for an acting leader to be appointed in the interim.
Labour's deputy leader Angela Rayner said the main opposition party aimed to trigger a vote of no confidence in parliament if the Tories did not get rid of Johnson immediately.
"He's a proven liar who's engulfed in sleaze and we can't have another couple of months of this," she told BBC radio.
Labour would need the support of dozens of Conservative MPs. But the strategy is fraught as it could trigger a general election, and the danger of Tory MPs losing their seats, if Johnson is defeated.
Johnson's spokesperson said there was no question of deputy prime minister Dominic Raab taking over as caretaker.
"The prime minister is acting in line with convention. He remains prime minister until a new party leader is in place and the work of the government will continue whilst that takes place," he told reporters.
A timetable for the leadership contest is expected on Monday, with the winner installed in time for the party's annual conference in early October.
Apart from Sunak, Conservative MP Tom Tugendhat and Attorney General Suella Braverman have also both officially announced their candidatures.
Former health and foreign secretary Jeremy Hunt, who lost to Johnson in 2019, was "virtually certain" to run again, a source close to Hunt told British media.
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