The news from Scotland this morning must have hit Downing Street with the kind of shock that Macbeth experienced when he was told that Birnam Wood had begun to move in the general direction of Dunsinane. “Ring the alarum bell!” he cried, as he saw the fulfilment of the witches’ prophecies of doom. “At least we’ll die with harness on our back.”

Tony Blair may not have used quite those words, but the sentiment was much the same. If today’s Populus poll in The Times is to be believed — and it echoes a trend that is beginning to grow steadily harder — then Labour is about to lose political control of a country that, for more than 50 years, it has regarded as its heartland.

A 10 per cent lead by the Scottish National Party would give it a seven-seat majority in the Scottish Parliament, enabling its leader, Alex Salmond, to form a coalition with the Liberal Democrats, or to chance his arm and govern as a minority party. Either way it would be a savage blow for Labour, which has regarded itself as the natural party of government in Scotland for as long as most voters can remember. It would be a damning verdict on Mr Blair’s premiership, on Gordon Brown’s economic policies, and on the devolution experiment so far. The prediction once made by Lord Robertson of Port Ellen that devolution would “kill nationalism stone dead” has backfired spectacularly.

A few things need to be said right away. First, this is a poll, not the real thing — and the election campaign is yet to begin in earnest; secondly, it is very far from a clarion call for independence — only 27 per cent of those polled actually want Scotland to leave the United Kingdom; nor is it a rebuff for devolution itself — 52 per cent would like the Parliament to have more power, not less; and Mr Salmond, for all the high recognition he enjoys, wins no better ratings, at 27 per cent, than his rival, the First Minister, Jack McConnell.

This is, primarily, a stinging rebuff to Mr Blair, who has succeeded in winning for himself a level of unpopularity that has not been witnessed north of the Border since Margaret Thatcher introduced the poll tax; in this case it is Iraq that has been the aggravating factor. For Mr Brown, too, the poll is grim news, because it suggests that he may take over as Prime Minister just as his homeland is turning its back on a decade of steady if unspectacular economic growth under his chancellorship.

More than that, however, it means that he will be fighting a political war on two fronts, taking on Tories south of the Border and Nationalists north of it. Mr Salmond and the SNP, if they do command a majority, are pledged to pursue the issue of independence from Day 1, thus provoking a constitutional crisis as well as a political one.

Mr Salmond is well aware that independence does not rank high on the Scottish agenda. But he believes that, once in power, he can promote it as the best option for the future of the nation. Within 100 days of being elected, he has pledged to introduce a White Paper setting out the terms of a referendum. He has even devised the question: “The Scottish Parliament should negotiate a new settlement with the British Government, based on the proposals set out in the White Paper, so that Scotland becomes a sovereign and independent state. Do you agree or disagree?”

He has said that he would hold the referendum towards the end of his first four-year term of office, by which time, he argues, the SNP would have proved itself in government. As only Westminster has the power, to run a full-blown referendum with binding conclusions, the Salmond version would have to take the form of a “consultative” process; but if passed it would be hard to ignore.

What can Mr Brown and Labour do to claw back the ground? The first thing, of course, is to win, not lose the election — and in the next four weeks we will see some of the most ferocious campaigning that Scotland has witnessed. On my mobile phone yesterday came the latest in a series of messages from Labour, promising revelations about “Wee Alex’s” dubious proposals, and we will see much more of this kind of thing as the party sets out to expose the weaknesses of the SNP’s tax-and-spend policies, and to warn the country about the dangers of separation.

But if the SNP really does go on to win the election, there is another way that Mr Brown could turn the situation to his advantage. For all that Mr Salmond’s commitment to a referendum is portrayed as a strength, it is in reality a serious weakness. The Lib Dems — the only serious contenders as coalition partners — have stated that they would refuse to form a government with the SNP so long as the referendum is part of their manifesto.

The Nationalists, clinging perhaps to the slimmest of majorities, would hardly have tasted power before they faced defeat at the hands of their opponents on a key political issue. Even if they did go on to hold a referendum, it would, if held in the present climate of opinion, almost certainly be lost. A “no” vote would not only be a public humiliation for the Nationalists, it would deprive them of their flagship policy.

Mr Brown, meanwhile, could present himself to the Scots and the English as the champion of a united Britain, thus winning back useful ground with those of his critics who are wary of his Scottish background. And he could do something even sharper — he could shoot the Nationalist fox between the eyes. Instead of waiting for Mr Salmond to come up with his consultative exercise, he could announce his own referendum Bill, which would introduce the real thing — a proper nationwide and binding poll asking the Scots what kind of future arrangement they really wanted.

And he could devise his own question, perhaps something along the lines of: “The Scottish National Party proposes that Scotland should leave the United Kingdom and go it alone. Do you agree or disagree?” You don’t need a witch to prophesy the outcome of that one.

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