The chances of the U.K. leaving the European Union have almost doubled in just three months, if the odds from Betfair Group Plc's gambling exchange are any indication of sentiment.

The probability of a majority vote for leaving the EU has jumped to 36 percent, from 18.5 percent at the end of July, based on the odds given to bettors on the outcome of the referendum.

While bettors are following the momentum of the polls, it would require a huge swing for so-called Brexit to become the favorite outcome.

"A vote in favor of staying in the EU is still the firm favorite at 1.56 (4/7 or a 64% chance), in much the same way as the Scottish Referendum market was predicting a No to independence from very early on," Betfair spokeswoman Naomi Totten said by e-mail. "The price for a vote in favor of leaving the EU is the shortest it has been since June, currently trading at 2.76 (7/4 or a 36% chance), but in the context of the market it is still very much assumed that Britain will vote to remain within the EU."

U.K. Prime Minster David Cameron  has said a referendum on Britain's membership will be held by the end of 2017 - though a date hasn't been set yet. The Conservative government wants the U.K. to stay part of a reformed EU and is currently in negotiations with regional leaders to secure chances before the vote.  

David Fuller's view

The entire civilised world (I aspire to qualify for membership of this group) should be hoping for a reformed EU which would make it significantly more entrepreneurial, competitive and democratic.  Unfortunately, we have grown older during a very different experience in terms of EU governance. 

I suspect EU bureaucrats will have varying degrees of ambivalence about the UK’s eventual terms for continued membership.  However, we will not get very far unless they are also interested in significant reforms for the overall benefit of their countries within the EU.  If not, why would anyone wish to maintain membership within an unreformable mediocrity?