Ladbrokes put the odds of the blonde bombshell leaving first at 4/1, while former leadership candidate Andrea Leadsom is second at 6/1, followed by eurosceptic Brexit secretary, David Davis, at 7/1.
Johnson's appointment was blasted by the French and German foreign ministers, who called him “a liar with his back to the wall” and denounced his behaviour as “outrageous”.
The internet was soon awash with tales of the former London mayor's most controversial foreign insults and gaffes. He once described George W Bush as a "cross-eyed Texan warmonger", and has called the President of Turkey a "terrific wankerer" (it rhymes with "Ankara", you see).
Jessica Bridge, spokesperson at Ladbrokes, said: "Time will tell if the Prime Minister's gamble with Johnson pays off.
"We expect the former London Mayor to stay the distance but should there be a wobble from any of May's recruits the early indicators point towards Boris leaving first."
May continued to build her first government yesterday, unveiling a raft of key appointments.
Greg Clark was given the new Department for Business, Energy and Industrial Strategy, while Damian Green is taking the work and pensions brief and Leadsom will head up the Department for Environment, Food and Rural Affairs.