Johnson, a leading Brexit campaigner who is seen as a possible successor to May, said Brussels would listen if she asked for the removal from the deal of the Irish "backstop", an insurance policy designed to prevent a post-Brexit hard border between EU member Ireland and British-ruled Northern Ireland.

Asked if she could stay on as leader and go back to the EU to renegotiate the deal if she loses the vote, Johnson told the BBC: "Of course, that is exactly what needs to happen."

"What people want to hear now is not stuff about leadership elections and personalities, what they want to hear is that there a plan to get out of this mess," he said.