The prime minister is to use a speech in Edinburgh to launch what he calls a positive vision for the union between Scotland and the rest of the UK.
David Cameron will argue that the current United Kingdom makes all its nations stronger, safer and richer.
A spokesman for First Minister Alex Salmond described Mr Cameron's views as a "threadbare case for an outdated union".
It comes as Mr Cameron is due to meet Mr Salmond for talks in Edinburgh.
Mr Cameron said the fight to save the union was now under way and as prime minister he would use all he has to win it.
He will tell an Edinburgh audience: "I think that the United Kingdom is better off together.
"Of course Scotland has to make this choice - but I'll be saying I hope you opt to stay in the United Kingdom.
"Together we're actually stronger. I think we have a fairer country, a better country, a richer country with all of us together.
"But I wouldn't ever threaten people in Scotland or say they can't do what they want to do. I'll just be appealing as someone who loves the United Kingdom, who loves our shared home"
A spokesman for Alex Salmond said: "He talks about the size of the UK economy, when the reality is that an independent Scotland would be the sixth-richest country in the developed world in terms of GDP per head - compared to the UK's 16th place."
The Scottish government has signalled its intention to hold a vote in the autumn of 2014, while the UK government has called for an earlier poll.
There are also disagreements over the wording of a question, the temporary transfer of legal powers to hold a referendum and whether a second question could be included on increased powers for the Scottish Parliament.
On his meeting with the Mr Cameron, Mr Salmond said he would be stressing to the prime minister that he should not try to "run the referendum" from London.
He told the BBC: "I think we should be open minded about a second question, I think if there's a huge upsurge of opinion in the unions, in civic Scotland, towards a second question then I don't think politicians should gainsay that process.
"Secondly I support the idea of 16 and 17-year-olds getting a vote in Scotland's future.
"Now let's see what the consultations throw up, the Westminster consultation, the huge response to the Scottish government's consultation."----
In: Regional News, Politics
Tags: UK, breakup, england, scotland, Cameron, stay, in, etc.
Location: Scotland, United Kingdom (UK/GB) (load item map)
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