But Robert Jenrick opened the door this week to a comeback for Mr Johnson, with the Tory leadership hopeful saying he would be “pleased” to welcome his ex-boss back as an MP.

Mr Jenrick also revealed he would be “delighted” should Mr Johnson wish to make a Cabinet comeback.

However, speaking to Mr Johnson’s father Stanley on GB News’s State of the Nation show, Sir Jacob Rees-Mogg said a future Tory leader could bring the ex-PM back now, even without a seat in Parliament.

Constitutionally, it is usually assumed that prime ministers must be MPs – but Sir Jacob agreed with Stanley Johnson’s assessment that such a rule does not always apply.

For example, in the 1960s, Alec Douglas-Home was briefly prime minister while seeking election to the Commons.

Sir Jacob said there were even fewer constitutional restrictions on non-MPs being Shadow Cabinet members, particularly a post like Tory chairman.

He added: “If Robert Jenrick wanted to make Boris Conservative Party chairman and Boris was willing to do it, there would be absolutely nothing to stop that.”

Since leaving top-flight politics, Mr Johnson has made millions of pounds giving speeches around the world.

He was also seen enjoying a holiday in Greece earlier this week, where he was spotted wearing a T-shirt with a potential cryptic message for fellow former PM David Cameron.