Surrey punter in line for £100,000 profit from what bookmakers describe as 'biggest political bet ever struck'

A man from Surrey has gambled £400,000 on Scotland voting no in the independence referendum, in one of the largest political bets ever placed.

Bookmakers William Hill said the customer presented a banker's draft for £400,000 at a branch in south-west London at odds of 1/4 for a no vote, potentially earning a profit of £100,000.

William Hill, which has already had a £200,000 bet placed on a no vote, said it had now cut its odds against a yes vote to 1/5 and slightly lengthened the odds of a yes vote from 11/4 to 10/3.

The bet came as an opinion poll carried out by TNS suggested that English support for Scottish independence has declined, as more English voters believed a yes vote would harm the UK. Backing for a yes vote has declined to 18% from 22%, while backing for a no vote has grown by eight points to 61%.

The referendum, which is being staged on 18 September, has already been described by bookmakers as one of the biggest political betting events outside a general election and a US presidential race.

Graham Sharpe, a spokesman for the bookmakers, said: "As far as we can ascertain, this is the biggest political bet ever struck.

"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.

"We are now well on the way to turning over £1m on the outcome of the Scottish referendum, which is exceeding our expectations."

He said the number of bets taken by William Hill on the referendum was split 55% in favour of a no vote and 45% for yes – broadly equivalent to the results of some recent polls.

The £400,000 punt has been matched by a very similar bet placed on the Australian federal election in September 2013, when an Australian gambler placed A$750,000 (£419,000) on the coalition led by Tony Abbott winning last year's federal election.

• This article was amended on 11 June 2014 to include the name of the company that carried out the opinion poll. It was further amended on 12 June 2014 because the earlier version referred to a bet on "the coalition government led by Tony Abbott retaining power"; in fact Abbott's coalition beat the incumbent Labor government.