A political punter stands to gain nearly pound(s)1 million if Scotland votes No to independence.
The wealthy man from Surrey has already staked a record pound(s)600,000 on the outcome of the referendum at a branch in south-west London.
However, according to bookmakers William Hill, he has added a further pound(s)200,000 to his wager, and now stands to collect pound(s)973,333 in the event of a No vote.
Spokesman Graham Sharpe said: "This gentleman's stake of pound(s)600,000 was already a record for political betting, but his latest 'investment' as he calls it, takes his total wager up to pound(s)800,000 - the biggest amount ever gambled on the outcome of a political event."
The man staked his initial bet of pound(s)400,000 at odds of 1/4 in June 2014, followed it up with an addition pound(s)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: "The client is a middle-aged gentleman who is not Scottish and does not have a vote in the referendum."