Scots and English would pay dearly for ending the union

The union between Scotland and England has a good claim to be the most enduring and successful international partnership in history, yet the atmosphere on the eve of its 300th anniversary is anything but celebratory. If opinion polls are to be believed, a mood of sourness appears to have taken hold, with a majority of voters on both sides of the border willing to entertain the idea that it might be better for Scotland and England to go their separate ways. With the SNP going into the Scottish parliamentary elections next May ahead of the field, it is not inconceivable that divorce proceedings could begin as early as next year. So is it curtains for the union?

In Scotland, polls indicating majority support for independence are nothing new. But separatism has never been anything more than a way of giving vent to a generalised sense of political discontent or protesting about specific issues such as job losses or the poll tax. This is additionally true today because Scotland's protest option of first resort - the Labour party - is now the main cause of voter disaffection. If allegiance to British power is weak, it is largely because it has become associated under Tony Blair's leadership with the shame and folly of Iraq and Lebanon. A government that restored Britain's self-respect as a force for good in the world would also do a great service to the unionist cause.

Scottish voters want to give New Labour a kick, but there is no reason to suppose that their underlying calculation about where their interests lie has changed. In the unlikely event that the SNP succeeded in forcing the issue to a referendum, its arguments would crumble under scrutiny. The SNP argues that an independent Scotland would be able to follow Ireland in slashing corporation tax to attract higher levels of inward investment. But Ireland also has the second-lowest levels of public spending in the OECD - 34% of GDP. Scotland has one of the highest, at more than 50%.

No amount of oil wealth could bridge this gap. For Scotland to emulate Ireland's "Celtic tiger" model would require an assault on public services far more brutal than anything inflicted by Margaret Thatcher. The SNP argues that cutting corporation tax rates would increase revenues. But what was true for Ireland almost certainly would not be true for a mature industrial economy like Scotland's. It is more likely that capital would flee and investors would be deterred as the reality of independence dawned. Whatever happens, England will remain Scotland's most important export market by far, and the creation of new barriers, such as a different regulatory regime and a new currency with a fluctuating exchange rate, could only impede trade to the detriment of both.

Scotland's voice would also be diminished on the international stage. Anyone hoping to make an impact on the global debates about the environment, trade and the war on terror would be in for a shock. Accustomed to the status and power conferred on them as part of the UK, most Scots can barely comprehend how marginal they would become.

This realpolitik calculation will always triumph over the romantic illusions and self-pity that comprise modern Scottish nationalism. But what of the English? The novel development in the constitutional debate is the rise of English national sentiment, with many voters appearing to favour a break-up of the UK. This new nationalism in many ways mimics its Scottish counterpart, especially in its capacity for self-pity. The complaint about being governed by an alien elite now centres on the supposed dominance of a "Scottish raj" in Westminster.

But the idea that the English are an oppressed majority is even more risible than the Braveheart fantasies of the SNP. There may be a disproportionate number of Scots in the cabinet, but it is "middle England" that calls the shots politically, which is why devolution became necessary to keep the union together. On any reasonable needs-based assessment, Scotland merits higher public spending because of lower population density and poorer health. Much the same applies to rural England, but rightwing opinion tends to excuse these particular "subsidy junkies".

In economic and financial terms, England certainly has less to lose from independence than Scotland. But the assumption that life would continue unchanged is wrong. England would become more dependent on energy imports, with consequences for its balance of payments. It would have to find somewhere else to base its Trident fleet, if indeed it was considered affordable on a smaller national budget. The British army, already overstretched, would become more so with the loss of Scottish revenues and troops. It is also plausible that Scotland's independence would precipitate the departure of Wales. Even Northern Ireland might not relish the idea of being a mere appendage.

England, like Scotland, would be a diminished force in the world - far more so than it seems willing to acknowledge. As a nation of 50 million, it would still be a big European country, but no longer the equal of France or Italy. Its influence would fall back even if it was granted the right to retain a permanent seat on the UN security council. English irritation at being portrayed as colonial oppressors, even after agreeing a fair devolution settlement, is understandable. It's time Scots took note and stopped behaving as if nothing has changed. But England must not succumb to the tawdry politics of false grievance. There is almost nothing that Scotland and England could do separately that they cannot do better together as part of the UK. The case for the union is therefore strong.

The real issue is one of political leadership. Can Labour persuade Scotland to end its flirtation with nationalism while doing more to address legitimate English concerns? More important, can a Conservative opposition desperate for power resist the temptation to abandon its unionist principles for a shallow, opportunistic and resentful English nationalism? It is in precisely this backlash that the real threat to the union lies.

David Clark, a former Labour government adviser.