DAVID Cameron's Government is targeting a "crushing victory" in the 2014 independence vote – to ensure the question is settled for a generation.
A senior Coalition source said attention was shifting from simply winning the referendum to ensuring there is no possibility of a so-called "neverendum".
However, the Scottish Government said the marked change in tone from the Tory-LibDem administration showed it was "rattled".
The Tory-LibDem Government is becoming increasingly confident it will win the popular vote. A series of opinion polls in recent weeks have put the proportion supporting independence at or below 30%.
However, last weekend another survey suggested a different outcome in 2014. It found 37% said they would vote Yes, rising to 52% if it looked like the Tories might remain in power at Westminster for another five years.
Senior UK Government sources admit they believe the vote will become closer than some polls would suggest as voting day nears. But they are also increasingly fearful that if the Yes vote is above 40% or even higher, then the SNP will press for another referendum within just a few years.
First Minister Alex Salmond has said the referendum is a once-in-a-generation event. However, a senior Coalition source said there was concern another vote would be too attractive for the SNP to ignore if the result was tight.
"If the Yes vote is 44%, would the SNP really settle for not holding another vote for a generation?" he said. "We are confident we will win this. But what we have to play for now is not just a victory but a crushing one."
He admitted it was hard to predict ahead of time what margin of victory would be needed to ensure there could not be another vote for a generation. "But that is what we now have to ensure," he added, "or else we will just keep having these arguments for years to come."
The Scottish Government accused the Coalition of being "rattled". A source said: "We are also aiming for a decisive victory and look forward to achieving a Yes vote for an independent Scotland.
"The Tory-LibDem Coalition are clearly rattled by the surge in support for a Yes vote shown by the latest polling – a surge which delivers a crushing victory for the Yes side when people are asked about the prospect of continued Tory government at Westminster."
The change in tone from the Coalition follows a week in which it had been keen to emphasise its "reasonable" approach to the referendum.
Last Monday, Mr Salmond and David Cameron signed an historic agreement in which the UK Government lent Holyrood the powers to hold the vote, expected to be held in 2014.
Yesterday, the Coalition published the Section 30 order which will technically hand over control to Holyrood.
However, the order still has to pass a number of parliamentary hurdles in Edinburgh and London. It is expected to be scrutinised by committees in both this autumn and winter with votes in Holyrood, the Commons and the Lords just before or after the New Year.
The timing is designed so the order will be agreed before it goes before a meeting of the Privy Council in February.
l The Tory-LibDem Coalition has admitted it would have to review its commitment to Trident if Scotland became independent.
The UK Government insists it is not making any contingency plans in case Scots vote to leave the UK. But asked whether the Government would continue with the ballistic missile nuclear deterrent "irrespective of the outcome of the Scottish vote", Defence Minister Andrew Robathan said independence would trigger a "review" of Trident.
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