Bookmakers believe Britain is likely to remain in the EU after this June's referendum.
The odds of Britain voting to stay in the EU are a general 1/3 at time of writing after David Cameron announced a referendum will take place on June 23rd.
William Hill spokesman Graham Sharpe said: "Bookies will see it as perhaps the biggest ever political gamble. Over £10m was gambled on the outcome of the Scottish Referendum, during the course of which one of our clients staked a world record political wager of £900,000 and called it right - the client is still considering whether to make another hefty wager on the outcome of the EU Referendum."
Meanwhile, Boris Johnson is the new favourite to be the next leader of the Conservative party, according to Ladbrokes.
His decision to back a Referendum leave vote saw him leapfrog Chancellor George Osborne to the top of the betting, with his odds shortening from 11/4 to 2/1.
The odds of the UK voting to leave the EU improved on the news, in from 12/5 to 2/1. That came after a weekend of almost non-stop betting support for Remain to win, following David Cameron's negotiation efforts in Brussels.
“The betting markets believe that Boris might make some difference to the Out campaign, but mostly that this improves his chances of succeeding Cameron at No. 10,” Ladbrokes spokesman Matthew Shaddick told BetOnPolitics.co.uk.
Ladbrokes latest betting
EU Referendum result
1/3 REMAIN
2/1 LEAVE
Next Tory Leader
2/1 Boris Johnson
3/1 George Osborne
8/1 Theresa May
10/1 Michael Gove
12/1 Sajid Javid
16/1 Priti Patel
25/1 Philip Hammond
33/1 Liam Fox