Embattled British Prime Minister Liz Truss on Thursday resigned as the Conservative Party leader saying she can no longer deliver the mandate she was elected on last month, ending her humiliating tenure at 10 Downing Street on her 45th day in the job following an open revolt against her chaotic leadership.

The 47-year-old outgoing prime minister will stay in charge until her successor is elected by the governing Tory party, with a speeded-up leadership election to be completed by next week.

Her former leadership rival, Rishi Sunak, is seen as a possible frontrunner in that race but a consensus within a divided Tory party remains elusive, with the Opposition Labour Party renewing calls for an immediate general election.

Ousted premier Boris Johnson is said to be considering another go in a race that is unlikely to have a clear choice, with former contenders Penny Mordaunt and Suella Braverman and UK Defence Secretary Ben Wallace among the names circulating.

"I recognise though that given the situation, I cannot deliver the mandate on which I was elected by the Conservative Party, Truss said in a brief statement outside 10 Downing Street.

The beleaguered leader said she had spoken to King Charles III to notify him of her resignation and also met the 1922 Committee chair Sir Graham Brady, in charge of Tory leadership elections, who has said a new Tory leader should be in place by next Friday.

"We have agreed that there will be a leadership election to be completed within the next week. This will ensure that we remain on a path to deliver our fiscal plans and maintain our countries economic stability and national security. I will remain as Prime Minister until a successor has been chosen, added Truss.

A downcast Truss, who stepped out of 10 Downing Street with her husband, said she had taken over at a time of great instability but ultimately admitted she had failed in her mission to deliver her economic agenda.

Her resignation on her 45th day in office makes her the shortest-serving Prime Minister in Britain's history. The second shortest serving prime minister was George Canning, who served for 119 days before he died in office in 1827.

Sunak, who lost out to Truss' now failed tax-cutting agenda in the leadership race last month, is seen as a key contender to step up to the post. But the picture remains extremely uncertain due to deep infighting within Tory ranks.

Loyalists of former prime minister Johnson believe the party should bring him back, given his solid electoral mandate from the 2019 general election. However, Truss' current troubles are themselves reminiscent of how Johnson was dragged out of office and forced to resign in early July amid an open revolt by a growing number of his MPs and ministers.

The latest turmoil comes a day after Home Secretary Braverman's explosive exit from the Cabinet after admitting a breach of the ministerial code by discussing government policy in private emails and a scathing parting attack on her boss.

"Not only have we broken key pledges that were promised to our voters, but I have had serious concerns about this government's commitment to honouring manifesto commitments, such as reducing overall migration numbers and stopping illegal migration, particularly the dangerous small boat crossings, the Indian-origin former Home Secretary wrote in her resignation letter.

Her exit came over what experts believe was a relatively low-level ministerial breach but reflects more serious differences between Braverman and Truss over the country's immigration policy.

The outgoing prime minister attempted to move on from the latest crisis by swiftly appointing Grant Shapps as the new Home Secretary. But coming just days after she sacked Kwasi Kwarteng as chancellor and brought in Jeremy Hunt who has since reversed all of her economic agenda, the latest Cabinet upheaval was widely expected to only speed up Truss' exit from 10 Downing Street.

To make matters even worse, the Speaker of the House of Commons ordered an investigation into reports of bullying and manhandling of some Tory MPs being made to vote with the government on an Opposition motion on Wednesday night.

Confusion erupted after Labour tabled a vote on whether MPs should get a say on the government's fracking plans to drill for gas. Conservative MPs were initially told the vote would be treated as a test of loyalty to the government, a so-called motion of confidence, and if they did not oppose the Opposition Labour motion they could face disciplinary action from the parliamentary party.

Chaotic scenes were caught on camera in the voting lobby of the Commons as whips tried to get Tory MPs to oppose the Labour motion. There were reports of further resignations from Truss' top team, which were later withdrawn.

The Labour Leader, Sir Keir Starmer, has renewed his call for a general election as the only way out of the pathetic squabbles within the governing party.

All the failures of the past 12 years have now come to the boil, he said at an event in Brighton, with reference to the Tory-led government's term in office.