Cameron's appeal to the 'silent majority' of Scots to stay within UK
The Prime Minister has said he would be left broken hearted if Scotland votes to become independent.
David Cameron travelled to Perth urging Tories to make a final campaign push to keep the country in the United Kingdom.
Mr Cameron stressed that the country's future is very much "an issue of the heart".
He told a rally: "It would break my heart to see our United Kingdom break apart," and he warned that a Yes vote would be "irreversible".
He said: "If Scotland walks away from the United Kingdom and votes for separation, it's not for a trial period, it would be permanent."
The PM said Scottish First Minister Alex Salmond and the SNP had failed to make a "really positive case" for independence, accusing them of leaving vital questions about Scotland's future after a Yes vote unanswered.
"It's for those who are arguing for this radical change to come up with... justification for why this is a good idea," the PM said.
"Why is it suddenly a good idea to make your sister working in Edinburgh or your brother working in London living in a foreign country?
"Why is it suddenly a good idea to split away from the organisations that have helped this country be everything it is in our world today?
"I don't think they have made a powerful case at all and I think they have left every single key question unanswered."
Mr Cameron said there is no certainty about what currency an independent Scotland would use, if it would be a member of both the European Union and Nato, and what the costs of leaving the UK would be. "None of the key questions have been answered," Mr Cameron insisted.
Yes Scotland chief executive Blair Jenkins said: "Mr Cameron's speech was billed as an appeal to the 'silent majority' – a term popularised by disgraced US president Richard Nixon, which is perhaps appropriate for a campaign that has called itself 'Project Fear'.
"The reality is there is a very vocal majority against David Cameron governing Scotland, with just a single Tory MP here – and we believe this will translate into a majority Yes vote on September 18."
The Yes campaign was quick to point out that David Cameron's speech was billed as an appeal to the 'silent majority' – a term popularised by ex-US president Richard Nixon. In 1969, the now-disgraced leader appealed for the backing of the 'silent majority' who didn't take part in rallies against the Vietnam war or sign up to 'flower power'. Both leaders appeared to want to contrast their faith in the realism of a quiet traditional majority with the perceived irresponsibility of a radical vocal minority.