David Cameron will vow to fight to keep the United Kingdom together, just hours before a key meeting with Scotland's First Minister on an independence referendum.
The Prime Minister will have talks with Scottish National Party leader Alex Salmond in Edinburgh.
The meeting comes after the First Minister discussed his plans for staging a ballot on independence with the Scottish Secretary Michael Moore earlier this week. However ahead of the talks Mr Cameron will use a speech in Edinburgh to make the case for the Union.
The Prime Minister will say: "The fight is now under way for something really precious: the future of our United Kingdom. I am 100% clear that I will fight with everything I have to keep our United Kingdom together. To me, this is not some issue of policy or strategy or calculation - it matters head, heart and soul. Our shared home is under threat and everyone who cares about it needs to speak out.
"Of course, there are arguments that can be made about the volatility of dependence on oil, or the problems of debt and a big banking system. But that's not the point. The best case for the United Kingdom is entirely positive. We are better off together."
A spokesman for Mr Salmond said it was important to discuss the country's constitutional future with the Prime Minister as well as Mr Moore. He said: "We look forward to the meeting with the Prime Minister - he takes the decisions on UK Government policy on the constitution and therefore it is important that we continue to discuss these matters with him, as well as the Scottish Secretary."
Mr Cameron will argue that Scotland was "stronger, safer, richer and fairer" as part of the United Kingdom.
Commenting ahead of the Prime Minister's speech today, Deputy First Minister Nicola Sturgeon told BBC Radio Scotland's Good Morning Scotland programme: "I am a patriotic Scot, and I accept that you can be patriotic and not support independence, but for me the argument for independence is not just one of the heart, it is one of the head."
Asked if she accepted there would have to a "leap of faith" to some degree, she said: "I think there is a wealth of evidence that says Scotland would be better off as an independent country, able to put our resources to work for the people of Scotland."
Earlier, Scottish Conservative leader Ruth Davidson told the programme Mr Salmond seemed to be using "fantasy figures" in his economic calculations. She said figures from 2010 showed that the amount of welfare payments paid in Scotland was more than three times the North Sea oil revenue for that year.