David Cameron will engage in a high-risk rebuttal of Scottish independence on Thursday, when he'll make a speech in Scotland which warns of the nation being marginalised at the UN Security Council and among NATO.

The PM will say: "We’re stronger, because together we count for more in the world, with a permanent seat on the United Nations Security Council, real clout in NATO and Europe and unique influence with allies all over the world."

In an impassioned speech in which Cameron will state: "It matters head, heart and soul" to him, the PM will say: "Our shared home is under threat and everyone who cares about it needs to speak out."

Number 10 was quick to clarify that there was no suggestion that a post-independence UK would lose its seat on the UN Security Council. But Britain's place there - considered increasingly tenuous given its relative wane as a global power - depends on its nuclear arsenal. How those weapons might be divvied up between England and Scotland will be a major bone of contention in the years leading up to the independence referendum.

Britain's fleet of Trident submarines are largely based in Scotland in order to defend the North Atlantic as part of NATO agreements. Many believe that if Scotland left the union, there would be no adequate alternative base for those submarines in England.

Alex Salmond is on-record as saying that an independent Scotland would not be a nuclear nation. Relocating Trident's existing bases to England would be expensive, and would lead to uncertainty on the current commitment by Britain under NATO to police international waters, particularly in the North Atlantic.

On arguments about economic independence for Scotland, David Cameron said: "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. Why? Well, first of all, let’s be practical. Inside the United Kingdom, Scotland - just as much as England, Wales and Northern Ireland - is stronger, safer, richer and fairer."

The long-running row on whether Scotland is really subsidised by England is likely to become fractious in the months to come. A study by the independent economic think-tank, the Centre for Economic and Business Research this week suggested that, on balance, Scottish oil revenues were cancelled out by the subsidy paid from London to Edinburgh under the Barnett Formula.

In traveling to Scotland to deliver a firm and personal riposte to rising sentiment there towards leaving the UK, Cameron is taking a significant risk. The PM - like all Tories - lacks popularity in Scotland. Although the Tories, Labour and the Lib Dems all oppose independence, Eton - educated Cameron is seen by many as a potential boon to Alex Salmond's campaign to win the referendum.

The SNP have made no secret of their belief that any involvement by Cameron only helps their cause to win the looming referendum.

Labour has considered drafting in former chancellor Alastair Darling as an ambassador for preserving the union, but it's far from clear who the Tories could deploy for their party.

A spokesman for Alex Salmond replied to the PM's comments on independence by saying: "The fact that the UK has Trident nuclear weapons of mass destruction dumped in Scotland – and wants to waste another £100 billion on a new generation ‘son of Trident’ system – is of absolutely no consolation to someone with a disability being deprived of their Disability Living Allowance."

The most recent poll on Scottish independence - released on the 5th of February - suggested Scots favoured staying within the UK by a margin of 5%.

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