A customer who has already staked a record £600,000 on the outcome of the referendum has added a further £200,000 to his wager, and now stands to collect £973,333 should the outcome of the poll be a No vote.
Spokesman Graham Sharpe said: "This gentleman's stake of £600,000 was already a record for political betting, but his latest 'investment' as he calls it, takes his total wager up to £800,000 - the biggest amount ever gambled on the outcome of a political event."
The man staked his initial bet of £400,000 at odds of 1/4 in June 2014, followed it up with an addition £200,000 at odds of 1/6 at the beginning of August, then added a third instalment on Saturday at odds of 1/5.
Mr Sharpe added: "All three bets have been placed in the same branch of William Hill in south-west London.
"The client is a middle aged gentleman who is not Scottish and does not have a vote in the referendum.
"We have now taken almost £2 million on the outcome of the referendum and stand to win over a million pounds should there be a Yes, vote, and lose a six figure sum should there be a No vote."