Last week, the London Mayor was at 3/1 to become the next PM in 2020, but after declaring his support for a Brexit he's now the bookies' favourite at 5/2.
Despite plenty of Boris bashing in today’s press, the bookmaker is banking that Johnson will come out of the referendum smelling of roses and in prime position to mop-up thousands of Eurosceptic grassroots activists.
Bad news for George Osborne, on the other hand. The chancellor has been robbed of his position as the favourite to succeed David Cameron, with a new offer of 3/1.
Plenty of punters still believe we're better together, though. This week Paddy Power received its largest "in" bet for weeks, where one Europhile put £10,000 on Britons choosing to stay in the EU.
The EU referendum is tipped to be a big moneymaker for Britain's gambling firms. Last week, William Hill predicted that betting on its outcome will outstrip that on the Scottish independence referendum, making it the "biggest political betting event ever" according to the firm.