A gambler in England has placed what bookmakers say is one of the world's biggest ever political bets, wagering £400,000 (€495,000) that Scots will reject independence in a referendum in September.

The middle-aged man, who has not been named, spoke with an English accent as he placed the bet in the prosperous southern county of Surrey near London.

The bet was made at odds of 1/4 on, meaning he will make £100,000 if he wins, said bookmakers William Hill.

"As far as we can ascertain this is the biggest political bet ever struck," said Graham Sharpe, media projects director at William Hill.

The bet is twice the £200,000 that was also placed on a "No" vote, staked by a Scottish man earlier in the independence campaign.

Scots will vote in a referendum on 18 September on whether to leave the United Kingdom.

Polls currently suggest roughly 40% of Scots will vote against independence and 30% will vote for, but many voters remain undecided.

William Hill said it had shortened its odds on a "No" vote to 1/5, suggesting this was now more likely to happen than previously thought.

Its odds on a "Yes" vote stand at 10/3.