Barack Obama, US President
William Hill and Ladbrokes tell IBTimes UK of US President's 'alarming' effect on betting oddsGetty

Barack Obama "drastically" shortened the betting odds for a 'remain' vote at the EU referendum, two of the UK's largest bookmakers have told IBTimes UK. William Hill and Ladbrokes said the US president's pro-EU intervention is one of the most significant developments in the campaign.

The price for a 'remain' vote at the 23 June ballot dropped from 4/9 (69%) to 2/7 (77%) with Ladbrokes, while William Hill saw its odds shorten from 4/9 to 1/4 (80%). Matthew Shaddick, head of political betting at Ladbrokes, told IBTimes UK: "The odds for remain have fallen drastically in the last three or four days, at least some of that you can put down to Obama."

He added: "The odds for remain have fallen drastically in the last three or four days, at least some of that you can put down to Obama. There have been an awful lot of people place quite a lot of money on 'remain' since last week."

Graeme Sharpe, a media relations director at William Hill, described Obama's effect on the EU referendum betting market as "rather alarming". "We were pushing along reflecting how the opinion polls were going, but since Obama's remarks – be it coincidental or not – 84% of the stake money we have taken is now for remain," he explained.

"The odds have shortened to their shortest level ever since we have been betting on the outcome of the referendum. The momentum has swung behind remain."

President Barack Obama makes emotional pro-EU plea to British voters during UK visitIBTimes UK

But both experts cautioned that the Obama effect could be short-lived on the market, with Boris Johnson's decision to back 'leave' in February shifting the odds towards a Brexit vote and Cameron's EU renegotiation deal moving the market towards 'remain' earlier that month.

Sharpe also explained that he expected more stakes to be placed closer to 23 June as punters seek to avoid unknown outside events having an effect on the vote. "We saw lots of swing in the Scottish referendum, it was going one way, then the other," he added.

"We've still got two months to go. There are an awful lot of people, who if they want to have a bet, are going to wait until as close to the kick-off as they can. It's a bit like waiting to know if you are going to bet on the football match if the star striker is injured or on a horse race where the going hasn't change to something the favourite doesn't like."

Latest EU referendum opinion polls

DatePollsterRemainLeaveUndecidedMethod
16-19 AprComRes51%40%9%Telephone
16-18 AprIpsos MORI49%39%8%Telephone
15-17 AprICM43%44%13%Online
13-17 AprORB53%41%11%Telephone
12-14 AprYouGov40%39%16%Online
7-11 AprTNS35%35%30%Online
8-10 AprICM42%45%12%Online