David Cameron has come under fire for negative campaigning over the case for Scottish independence after the Treasury produced figures claiming that Scots would be £1 worse a year off outside the United Kingdom.
At a joint press conference with his deputy Nick Clegg to mark the Westminster coalition's mid-term review, the prime minister said that winning the emotional battle over the ties that bind Scotland to the rest of the UK will be as crucial as the economic arguments in the independence referendum contest
Mr Cameron said there were "arguments of both the head and the heart" that needed to be made as he claimed Scotland would be worse off on its own.
But as well as the economic argument, he insisted that the campaign to preserve the union must also win the fight for the hearts of Scottish voters by showing "we are stronger together".
The £1 analysis, using an analysis of oil revenues over the course of devolution and produced by Treasury chief secretary Danny Alexander was intended to expose the SNP claim that people would be £500 better off a year as a "myth" but independence campaigners seized on the £1 cost as being a "price worth paying".
SNP Westminster leader Angus Robertson condemned the PM's comments.
"As the UK coalition's botched relaunch shows, the No campaign has started the New Year firmly on the back foot," he said.
"Danny Alexander's silly claim that an independent Scotland would cost people in Scotland £1 over a whole year had David Cameron floundering, and resorting to the old fears and smears that Scotland would be worse off with independence - even though the UK Treasury are no longer saying this.
"In the space of 24 hours, the No campaign has broken the Scottish Labour deputy leader's promise that they would fight a positive campaign.
"The reality is the latest figures show that Scotland is financially stronger than the UK as a whole to the tune of £2.7bn - or over £500 per person in Scotland."
Mr Cameron said he expected that the No campaign would be able to show "categorically" that Scotland would be worse off.
He said: "I think there are important arguments of both the head and the heart that need to be made in this great debate about the future of our United Kingdom and I profoundly hope that Scotland will vote to stay in the United Kingdom.
"I think when it comes to the arguments of the head, things like would Scotland be better off, I think we will be able to show, categorically, that Scotland would be worse off, would be less well off."
He said there would be a changing pattern as North Sea oil runs down and also said there would be uncertainty over jobs in the defence and financial services sectors if Scots voted for independence.
But Mr Cameron insisted the financial considerations were only a part of the decision facing the Scottish people.
"There are arguments of the head, but I profoundly believe we must win not only the arguments of the head but also of the heart: that we are better off together in the United Kingdom, there's a solidarity that we show each other, if different parts of the United Kingdom have a difficult time we are all there ready to stand behind those parts of the United Kingdom.
"We are stronger together, we are better off together, we are safer together.
"So those heart arguments will also, I think, win the day."
Mr Cameron praised Labour former chancellor Alistair Darling, who is leading the No campaign, for doing a "fantastic job".
But Mr Robertson mocked the PM for failing to remember wich campign he was backing.
"Embarrassingly, the prime minister couldn't remember the name of the No campaign - first calling it the 'Yes campaign', and then 'Alistair Darling's campaign'," he said.
"Since David Cameron and Nick Clegg's infamous rose garden media appearance, the Westminster government's promises on issue after issue lie in tatters.
"Pledges on meeting borrowing reduction targets, on reversing years of decline to Scotland's defence footprint and on reforming the House of Lords - to name but a few - have all been abandoned.
"The coalition's track record has been an appalling one and people are understandably fed-up of decisions on key issues affecting Scotland being made by a Westminster Government that has been rejected by people in Scotland.
"Decisions affecting Scotland should be made by people in Scotland, who by definition care most about getting them right.
"Only a Yes vote in next year's referendum will give us that opportunity and ensure that Scotland is no longer paying the price of being tied to a failing Westminster system."