A woman in London has placed a £100,000 bet on Britain staying in the EU

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Prime Minister David Cameron is leading the campaign for Britain to remain an EU member state.

A London woman has placed a £100,000 bet on Britain voting to remain in the European Union in the June 23 referendum.

The unnamed William Hill client - who according to the bookmakers lives in central London - stands to win a total return of £140,000 after backing Britain to stay in the 28-nation bloc at odds of 2/5.

Incredibly, it's the first bet the female client has ever placed, according to William Hill's Graham Sharpe who spoke to Business Insider on Monday.

Sharpe told BI in an email: "It was her first, and, so far, only bet she told me when I spoke to her on Saturday."

He added: "This is the biggest political bet ever placed by a female client, but some way short of the biggest ever political bet, which is one we took on the Scottish Referendum from a male who staked £900,000 that the Scots would opt for the status quo - winning a profit of £193,333.33."

"I have spoken to this gentleman recently, and he is currently weighing up his options before deciding whether to 'reinvest' on the EU Ref!"

Sharpe claimed that William Hill received £3.25 million worth of bets on the Scottish referendum two years ago. However, he believes that the EU referendum is on track to breaking that record.

Fellow bookmakers Betfair made a similar prediction last month. The betting company told BI that the June referendum would likely to Britain's biggest political betting ever.

"Six figure bets are not uncommon in politics," Sharpe told BI.

"Before the £900,000 bet, the largest we had taken was £200,000, but we have taken a number of those - one of them on David Cameron becoming Tory leader back in the day."

"I will be very surprised if we don't take more six-figure bets before June 23."

William Hill's current odds indicate that the probability of a Brexit is just 22% (7/2). This forecast isn't far away from that of polling analyst Matt Singh - whose latest forecast indicated that the likelihood of Britain voting to leave is as low as 17.7%.

Sharpe told BI that William Hill has received bets on the EU referendum for all over the world.

"For Remain, we have taken €1000 from an Austrian client; €59 from a Finnish gentleman; 400 yen from a Japanese customer; €50 from a Maltese punter, $580 from a Russian gambler, and $400 from a Ukrainian client."

"It seems there is a growing global interest in EU Ref betting," he added.