US Edition Leading financial spread better IG has put the chances that Scots will vote to remain in the Union at 81pc.
It said a survey of other spread betters and investor behaviour on its own platform indicated that investors believe the referendum is still far more likely to end with a 'No' vote.
Brenda Kelly, chief market strategist at the broker, said a straw poll of nearly 200 of IG's clients put even more weight behind the 'No' camp, with 88pc saying they think the Scots will stay.
IG's so-called 'binary trade' on Scottish independence, where investors bet on either a 'Yes' or 'No' outcome in this week's referendum, currently places far more probability on the 'No' vote.
But David Madden, market analyst at IG, said the Scottish referendum binary is "by far one of the most actively traded, and [with the] most balanced flow" that the spread better had ever seen, meaning that investors' predictions for the outcome are more deeply split than in past political events.
"Previous political outcomes have been quite one way," he said.
Jasper Lawler, market analyst at CMC Markets, said that most major clients have told him they expect the 'No' camp to win out. But he added that higher levels of hedging betrayed investors' deep uncertainty around the outcome.
"Clients are using different instruments to trade on different outcomes to hedge themselves," he said. "You only do that if you're unsure".
Unlike IG, CMC Markets is not offering a trade specifically on the outcome of the referendum, but investors are placing bets on sterling and the stock markets as a proxy.
Mr Lawler said investors were placing bets that the stock market will slide since the potential fall in the FTSE 100 should the Scots head for the exit is much greater than the lift it would experience if they stayed put.
"Maybe what’s going on is you have positions in both where the general expectation is that it will be a 'No' but [investors] put a trade on 'Yes' because the risk reward would be so lucrative," he said.
IG's Mr Madden also told the Telegraph that the broker had seen three to four times more trades made on the Scottish referendum binary than any political binary they had run before.
Trading volumes spiked in the last week and a half, he said, ever since a YouGov poll put the 'Yes' camp ahead 51-49 for the first time.
Subsequent polls have since returned the lead to the 'No' vote, but in every case, there are too few points in it to make a confident prediction.
IG has set the mid-price for a 'Yes' vote at 20 points, with the trade settling at 100 points should the Scots opt to leave the Union. This means investors stand to make big earnings should the Scots opt to leave the Union. Conversely, it has set the mid-price for a 'No' vote at 80 points, with the trade settling at 0 points should Scotland in fact vote 'Yes'.