A punter has staked £400,000 on the independence referendum resulting in a No vote.
The middle aged man placed the bet via a banker's draft at odds of 1/4 when he visited a William Hill's betting shop in south-west London. He stands to make a profit of £100,000 if correct.
The bookies said it was believed to be the largest political bet ever in the UK, or even the world.
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Few details have emerged of the punter, other than that he's from Surrey and does not have a Scottish accent.
Hill's spokesman Graham Sharpe said: "The previous record was a £200,000 bet also struck on a No vote in the Scottish Referendum earlier in the campaign, and the same amount was gambled by a customer who backed David Cameron to become Tory leader."
Hill's have now shortened their odds on a No vote from 1/4 to 1/5, and are offering 10/3, out from 11/4, on a Yes vote.
Mr Sharpe said: "We are now well on the way to turning over £1million on the outcome of the Scottish Referendum which is exceeding our expectations."