Tony Blair has urged voters not to "rip Scotland out of the UK", saying full independence would be disastrous.
Asked about a poll which suggested 51% of Scots favoured independence, Mr Blair said he would have to persuade them otherwise.
He blamed nationalists for trying to claim voters were only "true Scots" if they were backing independence.
But Scottish National Party leader Alex Salmond said Mr Blair was "out of touch with public opinion in Scotland."
SNP 'folly'
Mr Blair told his monthly press conference: "To end up having a separate Scottish currency, separate Scottish armed forces, separate Scottish economy when we are so integrated together would be disastrous for people.
"I think that when this debate progresses, it's not just that people will realise the folly of the SNP position."
Six months before the Scottish parliamentary elections, polls suggest the SNP is neck and neck with Labour - which rules in coalition with the Liberal Democrats.
"What's in the best interest of Scotland is to have a strong Scotland within the United Kingdom"
The SNP has promised a referendum on independence within four years, should it win power.
Asked about a poll in the Scotsman paper at his monthly press conference on Monday, Mr Blair said it showed he would have to engage with Scottish voters, to persuade them of the benefits of remaining in the UK.
Shared trade
But he said increasing Holyrood's powers was not the answer.
"What I think it's about is the attempt by the SNP and others to say: 'You're only truly Scottish, in inverted commas, if you are making the case for independence," he said.
"But that's rubbish. The question is what's in the best interests of Scotland and what's in the best interest of Scotland is to have a strong Scotland within the United Kingdom, where you have the benefits of devolution.
"You take your local decisions in relation to law and order, education, health and so on, but at the same time you get the benefits, as we do in England, of being part of the UK."
He said both England and Scotland benefited from shared trade, currency, armed forces and social security systems.
"You rip Scotland out of the UK, which is the SNP policy, you will lose those benefits," he said.
"You will end up with an uncertain economic future with less power for people in Scotland to effect the big changes that are happening in the world."
In response, Mr Salmond said: "The more the prime minister attacks independence, the more popular it becomes. I urge him to continue in a similar vein.
"Labour's negative smears and fears campaign simply isn't working in the face of the SNP's positive case for Scottish independence."
He said most Scots wanted more powers for Holyrood and said there was a "clear consensus" for an independent Scotland.
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