Former prime minister Sir John Major has said it would be “unwise and may be unsustainable” for Boris Johnson to remain in office while a new Tory leader is elected.

Sir John warned Mr Johnson would continue to have the power of patronage and the ability to make decisions affecting the lives of people across the country despite losing the support of his MPs and ministers.

He warned the new interim Cabinet appointed by Mr Johnson following the wave of resignations this week may not be able to “restrain him”.

In a letter to Tory 1922 Committee chairman Sir Graham Brady, Sir John said: “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.

“In such a circumstance the Prime Minister maintains the power of patronage and, of even greater concern, the power to make decisions which will affect the lives of those within all four nations of the United Kingdom and further afield.

“Some will argue that his new Cabinet will restrain him. I merely note that his previous Cabinet did not – or could not – do so.”

Sir John suggested Deputy Prime Minister Dominic Raab could serve as acting prime minister until a new leader is elected.

Or he said Tory MPs could elect the new leader who would become prime minister, with party members then asked to endorse the decision.

Sir John said: “Neither of these options is ideal, but the interests of the country must be given priority over all else and with so many long-term and critical issues before us, an imaginative response even at the risk of some bruised feelings within the party is most definitely in the national interest.”

Under the expected timetable, Conservative MPs will take part in a series of votes to whittle leadership candidates down to two, with Tory members then deciding the winner.

The process could take months, with a new leader expected to be in place before the party conference in October.