Former UK Prime Minister John Major said Boris Johnson should be replaced immediately rather than be allowed to remain interim leader, as his cabinet may struggle to "restrain him."

Johnson announced his resignation as prime minister on Thursday after he was deserted by cabinet ministers and many Conservative Party lawmakers who said he was no longer fit to govern. 

Outside his Downing Street office, Johnson announced his "painful" resignation. Still, he defied pressure to step down immediately, insisting he planned to stay prime minister while his party picked his successor.

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Major, who served as Prime Minister from 1990-97, released a letter saying it was "unwise, and may be unsustainable" for Johnson to remain in power for several months.

That call was echoed by several Conservative members of parliament (MPs), who said Johnson's behavior this week when he refused to resign, means he should be forced out before the leadership contest, a process that could take months.

"The proposal for the prime minister to remain in office - for up to three months - having lost the support of his cabinet, his government, and his parliamentary party is unwise and may be unsustainable," Major said in a letter.

"Some will argue that his new cabinet will restrain him. I merely note that his previous cabinet did not - or could not - do so."

The prime minister's decision to quit marks the end of a rollercoaster political career in which he led Britain out of the European Union and took his Conservative Party to the largest election victory in three decades.

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It follows three years of scandals, including anger over parties held at his Downing Street office during coronavirus lockdowns, accusations of breaking international law over his threat to override parts of the treaty governing Britain's exit from the European Union, and, most recently, his handling of sexual harassment allegations against a lawmaker.

'Tarnishing reputations'

Tim Bale, a professor of politics at Queen Mary University in London, said that British prime ministers ousted outside of general elections are typically allowed to remain in power until a successor is chosen.

But Bale said there was a lack of trust between Johnson and his MPs after he refused to step down this week, even after several cabinet ministers told him to quit.

"A lot of MPs simply don't want to risk him saying and doing anything over the summer that will tarnish the party's reputation further," Bale said.

Simon Hoare, a Conservative MP, said Johnson's behavior meant he had forfeited the right to remain as a caretaker leader.

He blasted: "Ministers resigned because of the PM. The party lost confidence because of the PM. It is beyond credulity that Mr. Johnson can stay in office... He has to go, and go means go."

Dominic Raab, the deputy prime minister, would be a suitable temporary replacement, Major and several Conservative lawmakers said.