Pollsters TNS-BMRB, formerly System Three, have conducted eight identical surveys since August 2007 and the pro-independence camp, having almost closed the gap on its opponents for much of that period, now finds itself back where it started at 15 points adrift.

Labour’s culture and constitution spokeswoman Pauline McNeill said: “This poll clearly shows that support for independence is collapsing in Scotland. When Scots are focusing on the economy, jobs and Scotland’s education crisis, Alex Salmond’s SNP Government is obsessed with the constitution.

“Poll after poll is showing support for separation crumbling but this is the eighth in a series asking the same question and the findings of it will not make happy reading for Mr Salmond.”

Deputy First Minister Nicola Sturgeon pointed out that sampling preceded last week’s White Paper launch and insisted that the “enormously encouraging” 31% of people backing independence was the highest finding in any poll since May, while the 15-point gap compared well with another recent poll that saw independence support trailing by 28 points.

She added: “We are confident that we can bridge the remaining gap in a successful independence referendum campaign. The Government has the confidence to test independence in a free and fair referendum.

“In contrast, the London-based parties are running scared of the people having their say on Scotland’s future.”

Scots Tory leader Annabel Goldie said: “This is another terrible setback for Alex Salmond and the SNP. Mr Salmond should stop wasting millions of pounds of taxpayers’ money on his obsession with an independence referendum, ditch his doomed Bill and get on with the job people elected him to do.”

Scottish Liberal Democrat leader Tavish Scott MSP said: “People judge governments on whether they deliver on their promises on the issues that matter to them. The SNP are failing Scottish education and the number of people out of work in Scotland is rising. That is why this Government is losing support.”

The most recent TNS poll of 998 voters was carried out in 55 constituencies between November 25 and December 2. Like all the others it used the wording initially intended by Mr Salmond for a referendum: “I agree/do not agree that the Scottish Government should negotiate a settlement with the Government of the UK so that Scotland becomes an independent state.”

The result this month saw those agreeing with independence fall to 31%, a five-point drop since May and the lowest recorded over the series. Over the same period the number disagreeing with independence rose seven points to 46%, the highest since the first snapshot.

This means that over the summer and autumn the gap has widened from three points to 15, a measure of the difficulty Mr Salmond would face in winning his argument. Nor has the Government’s National Conversation brought clarity to public opinion, with the numbers saying they don’t know how they would vote in a referendum far higher in the most recent two polls than they were at the outset.

Chris Eynon of TNS-BMRB said of the recent shift: “Even with 23% undecided, this leaves no doubt about the current mood of the Scottish people on this issue and calls into question the determination of Alex Salmond to press ahead with his plans for a referendum.

“When taken along with last month’s voting intentions figures, it would appear that the Scottish electorate are falling out of love with the SNP and their policies, at least for the time being.”