The No camp high command expressed serious doubts for the first time about whether the Union can be saved after the YouGov survey revealed its lead had been cut by eight points in one month.
With just 15 days to go, one senior Better Together source admitted the latest poll showing a 53/47 split accurately reflected Scotland's mood.
The insider said: "The ground has shifted a lot in the past week. I still think we'll win, but it's close and it's possible we'll lose. Yes can win from here; that's a shift in my thinking. I really never thought we would lose, now I'm thinking - it's possible we might."
He added: "The Yes campaign has brought Scotland to the edge of the precipice and the country will collectively look over the edge and say: 'Let's go for it,' or 'That's scary, let's not do that'. Neither camp is sure what will happen."
At Westminster yesterday, No 10 made clear it was not pressing the panic button.
After Alistair Carmichael, the Scottish Secretary, updated the UK Cabinet on the campaign, David Cameron's spokesman insisted: "There isn't a change here in the approach that the Government has been or will be taking, which is, in summary, real confidence in the argument that the Government and others are making."
He stressed there would continue to be a "consistency of approach" and there was huge commitment and energy being put into the campaign by the Coalition Government. The Prime Minister is not due in Scotland this week but is expected next week.
As one senior Whitehall insider predicted the final two weeks would be "a rollercoaster", Better Together leader Alistair Darling noted: "Earlier this year people would say to me - it's in the bag, what have you got to worry about? I always said then, as I do now, no it's not. It's going to be close and your vote could make the difference. So a tightening of the polls helps us."
But the markets responded nervously to the latest snapshot of opinion. Sterling fell to near a five-month low against the dollar and also slipped versus a generally weak euro, while the cost of hedging against sharp swings in the pound rose as investors sought to insure against what they regarded as the risk of independence.
During a visit to Eden Brewery near St Andrews, the First Minister described the polls as "very encouraging" and declared: "I have always thought we would win."
But he insisted it was the reaction on the streets that was most encouraging. "Dundee yesterday was extraordinary. It was a carnival atmosphere with certainly hundreds of people in the city centre. People were queuing to register to vote ... this is a democratic sensation we're having," hailed Mr Salmond.
Last night, Danny Alexander, the Chief Secretary to the Treasury, dropped what he called an "EU currency bombshell" by revealing correspondence from Olli Rehn, who recently stepped down as European Commissioner for economic and monetary affairs, in which he said combining a policy of sterlingisation with EU membership would "simply not be possible".
"So, an independent Scotland would face a simple choice: using the pound like Panama uses the dollar or joining the EU," said the Highland MP.
But the First Minister brushed Mr Alexander's claim aside, saying: "We believe there will be a commonsense agreement on a common currency."
Today, as Mr Darling is due to speak at the Oil and Gas UK Conference in Aberdeen, a report is published estimating Scotland could be sitting on more than double the amount of oil and gas reserves currently predicted. The study, from oil and gas industry jobs board oilandgaspeople.com and independent North Sea experts, suggests the West Coast alone could provide oil and gas for at least 100 years with an estimated value of more than £1 trillion.
Meanwhile, banking industry sources said Lloyds executives were considering moving the group's registered office from Edinburgh to London in the event of independence and Labour's Thomas Docherty suggested he would stand down as a Fife MP at the 2015 General Election if there were a Yes vote.
Meanwhile, Nicola Sturgeon, the Deputy First Minister, and Douglas Alexander, the Shadow Foreign Secretary, appeared in an STV referendum debate.