Almost 75% of bets placed so far on the outcome of the EU referendum have been for the 'Leave' option, according to bookmakers William Hill.
William Hill spokesman Graham Sharpe said: "Since Boris Johnson entered the argument on the 'leave' side we have seen an upsurge of betting interest on that outcome and have been forced to cut the odds twice. The Boris factor has certainly boosted those betting on Brexit."
Meanwhile, David Cameron has been cut from 10/1 to 7/1 to leave his position this year.
"If the Prime Minister loses the EU referendum argument there is a strong case to be made that he might stand down as Tory leader sooner rather than later," Sharpe added.
That news comes after Boris Johnson's odds of replacing Cameron as PM this year were cut by Ladbrokes, who rate Boris a 10/1 shot (from 20/1) to stride into Downing Street at any point in 2016, as the 'Bojo factor' also triggers a rush of Brexit bets forcing the odds of a leave outcome into 15/8, from 12/5.
Ladbrokes' Alex Donohue told BetOnPolitics.co.uk: "Political punters can't get enough of Boris and fresh from backing him into favouritism for party leadership, his fans are now betting he'll have his bosses' job before the end of the year too."
He added: "We've never known one politician with his power when it comes to moving markets. Every time he opens his mouth we reach for our chalk."