Gambling company Betfair said on Tuesday it was already paying out winnings to customers who had staked money on a "No" vote to independence.
Bookmakers sometimes pay out early on sports events when they consider the result a foregone conclusion, deciding that the publicity they generate is worth the risk should there be a surprise turnaround.
"We've decided that 'No' is most likely to be the winning vote in three days' time, so we've put our money where our mouth is," said Betfair, an online betting specialist.
The referendum has attracted record levels of interest from British gamblers.
One London-based gambler will collect £973,000 if Scotland rejects independence after staking £800,000 on a "No" vote with William Hill, Britain's largest bookmaker.
Opinion polls indicate the vote will be very close. Out of four recent polls, three showed those in favour of maintaining the union had an advantage of between two and eight percentage points.
But an ICM poll conducted over the Internet showed supporters of independence in the lead.
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