Former British Prime Minister John Major has said Boris Johnson should be replaced immediately rather than remain interim leader because his cabinet may struggle to “contain” him, among other calls to remove him from office.

Johnson announced his resignation as prime minister on Thursday, after being dumped by ministers and many Conservative Party lawmakers who said he was no longer fit to rule.

Speaking outside his Downing Street office, Johnson announced his “painful” resignation, but defied pressure to resign immediately, saying he planned to remain prime minister while his party chose a successor.

Major published a letter stating that Johnson’s stay in power for several months was “reckless and unsustainable”.

That call was echoed by several Conservative Members of Parliament, who said Johnson’s behavior when he refused to step down this week meant he needed to be sacked before a leadership contest, a process that could take months.

“The proposal that the prime minister stay in office for up to three months is reckless and untenable, as he has lost the support of his cabinet, government and parliamentary party,” Major said in the letter.

“Some will argue that your new cabinet will include this. I just note that your previous cabinet did not – or could not – do this.”

The prime minister’s decision to resign marks the end of a tumultuous political career that drove Britain out of the European Union and led the Conservative Party to its biggest electoral victory in three decades.