The Prime Minister draws pointed contrasts between the discipline Labour adopted when drawing up its policies in opposition in the 1990s and the SNP's approach to the prospect of forming the next Scottish Executive, which he says lacks the economic credibility needed to persuade voters.
"It is not just that economic competence is vital for the prosperity of families and society as a whole. It's also that the rest of your programme means nothing if you can't afford to fund it," Mr Blair writes.
"In opposition, Gordon Brown and I put such a strong emphasis on establishing Labour's economic credibility when we were seeking to win the trust of the electorate."
�The biggest challenge, in fact, for the SNP is not showing how separation will enable them to spend more, as they say they will. It is how they will continue to find the cash to match Labour's spending now.
For Scotland , like Wales , Northern Ireland and many rural parts of England , already receives higher public expenditure per citizen than the UK average. According to the SNP's own figures, there is a �10 billion difference between money spent in Scotland and money raised. That is �4,000 for every family in Scotland met from the UK Exchequer.
This is a simple recognition that it costs more to provide the same standard of infrastructure and public services in areas with more scattered populations. Scotland, with a third of the land mass, close to half the coastline, a large network of island communities but less than 9 per cent of the UK population, certainly qualifies.
But break up the UK then this Union dividend disappears. So before the SNP start to explain how they are going to find the �1.7 billion extra needed to write off student debt, or the additional �1.4 billion to fund the promised cut in corporation tax, they had better explain how the �10 billion shortfall is going to be met.
The SNP answer, of course, is their proposal for an oil fund. Over ten years they say it might reach �90 billion, allowing Scotland to live off the interest. This rests on two large and uncertain assumptions - that oil prices will remain high and production will continue at present levels. It seems a very big gamble, for example, to base your economy policies on a single commodity whose price has fluctuated by a third in recent months and by much more in recent years.
It is not surprising, therefore, that a nationalist MP hinted on the radio the other day that they might have to raise personal taxes. It would be hard-working Scottish families who would pay the price for nationalist recklessness, and it is time the SNP owned up to it.
There is no reason to take these risks, no reason to surrender the strength and stability the UK delivers to all its nations. Scotland , despite all the challenges still to overcome, is prospering within the Union . As the world becomes interdependent by the day, it's a modern partnership, which is working well for all parts of the UK - at home, within Europe and across the world.
All this will be at stake again in the elections next May. Scotland, not for the first time, will have a choice of direction. Twice before Scots have rejected the idea of ripping Scotland out of the UK . I believe the Scots, who know more than most about the importance of community, will again decide that we are stronger together than we are apart.�