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A study by the University of Stirling estimates a 79 percent chance that the people of Scotland will reject the chance to end a 307-year-old union with England, up from 70 percent in April.
Earlier in the year some polls had shown the pro-independence Yes campaign closing the gap on the pro-union campaign's lead.
However, a YouGov poll for the Times newspaper found that backing for the anti-independence campaign had risen, with the Yes campaign struggling to convince voters a split would not hurt the Scottish economy.
Professor David Bell, who looked at betting trends among bookmakers for the University of Stirling, said the chance of a No vote on Sept. 18 probably remained between 70 and 80 percent throughout the first half of the year, even as polls fluctuated.
"So long as punters do not think there is much chance of a sudden swing in voting intentions, their money will go increasingly towards the leading campaign," he said.
The YouGov survey found that backing for the anti-independence campaign had risen by a point to 54 percent since a similar poll two weeks ago, with support for nationalists dipping to 35 percent.
However, the campaign in favour of independence cited "three other recent polls (that) have recorded a significant increase in backing for Yes...
"Of course, the polls will fluctuate from time to time," a spokesman for Yes Scotland said.
Both sides have stepped up campaigns to emphasise the economic benefits or disadvantages of independence.
If Scotland votes in favour of independence, Edinburgh would have to negotiate with London on issues ranging from security to the status of the British pound in Scotland and division of assets, including energy reserves.
Nationalists maintain that, in the case of a Yes vote, the UK government would have to agree to some form of currency union, although the UK government insists an agreement to share the pound is not on the table.
Professor Bell said that between mid-March and mid-April, the chances of Scotland voting for independence improved, in part as a reaction against the UK government saying an independent Scotland could not share the pound.
The major British parties have since changed tack, and are campaigning more vocally for a No vote in favour of greater devolution of powers within the United Kingdom.
On the issue of the economy, the YouGov poll suggested the No campaign was winning the argument with 49 percent believing Scotland would be worse off under independence, up 4 points from three months ago.
Just 27 percent said that Scotland would be better off under independence, a drop of three points since March.