Any Conservative MP hoping to replace Liz Truss as Prime Minister needs the support of 100 colleagues by Monday.
Will Walden understands Mr Johnson's cutting short a holiday abroad.
No one's formally declared yet, but Rishi Sunak and Penny Mordaunt are considered frontrunners.
Liberal Democrat leader and Kingston MP Sir Ed Davey is calling for MPs to do their “patriotic duty” and support an election.