Punters have suddenly started to bet heavily on a Brexit -- and bookmakers have no idea why

Matthew Shaddick, LadbrokesInternational Business TimesMatthew Shaddick, head of political betting at Ladbrokes.

British punters have suddenly started to pile money on Britain voting to leave the European Union on June 23, and a betting expert can offer no explanation as to why this is happening.

Matthew Shaddick, head of political betting at Ladbrokes, told an audience at an event attended by Business Insider on Wednesday night that the betting company had seen a huge increase in the number of individual bets being placed on a Brexit this week.

This odd development in the world of political betting comes with less than four weeks to go until Britons will vote on whether the country should remain an EU member state.

Since Monday, 47.9% of the bets Ladbrokes has received on the EU referendum have backed Britain to leave the EU.

In the five weeks previous, however, the share of bets placed on a Brexit didn’t even surpass 16.1%, as shown in the chart below.

This means that the number of bets placed on a Brexit has nearly tripled.

Speaking at the Institute of Directors in central London, Shaddick said: “There’s been no major political events or developments in the campaigns to explain this move. I don’t have a clue.”

He added in an email to BI on Thursday: “This week has seen a notable increase in the amount of money being bet on Leave. At last, we’re beginning to see some more confident punters willing to stake hundreds and, in some cases, thousands of pounds on a Brexit outcome.”

One reason why betting behaviour has changed so dramatically this week might be the YouGov and ICM online opinion polls published on Monday that said that the race between the two campaigns was exactly neck-and-neck.

The results of opinion polls conducted online continue to show a really close contest — despite phone polls, analysts, and bookmakers indicating leads for Remain ranging from steady to very commanding.

It’s possible that Britain’s punters feel that there’s a realistic possibility that the phone pollsters and analysts are misreading public opinion and online pollsters are being more accurate.

What is also interesting is the huge difference between the average size of bets placed on Remain and Leave. The average worth of bets placed on Leave so far has been £60, but for Remain the amount has been a much larger £343, according to data Shaddick sent to BI.

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