Bookmakers dramatically reversed the odds on Britain leaving the European Union on Friday as early results from a historic referendum pointed to strong support for a Brexit.

Just hours after giving odds that indicated an estimated 90-percent chance of Britain staying in the EU, the main betting companies shifted to make a "Leave" vote the strong favourite.

Betting exchange Betfair moved to odds of 8/13, an implied probability of a Brexit of more than 60 percent. Ladbrokes had odds of 4/7, an implied probability of 63 percent.