It was the late Donald Dewar who said that devolution in Scotland was “a process, not an event.” He meant that the 1998 settlement which gave the new Scottish Parliament a wide range of powers, while Westminster retained control of economic and foreign policy, would, inevitably, evolve over time. He could not have envisaged how rapidly that process would accelerate, nor would he have dreamt that, less than ten years later, a Nationalist government, pledged to securing independence, would be running the country.

For Gordon Brown, for whom the Union is central, and Britishness a cherished notion, it must be a special nightmare to see his old political foe Alex Salmond holding sway in Scotland — his home, his hinterland, and, formerly, his power base. He has had to witness the withering of

Labour’s hitherto impregnable control in Scotland, the loss of its leader, Wendy Alexander, and the stunning defeat in Glasgow East, which demonstrated that even here, on its home territory, the old loyalties no longer hold.

Quite how to turn the tide has so far defeated him and his Scottish advisers. Until now, the Brown line has been to oppose, in public at least, the idea of giving Holyrood any tax powers beyond its present ability to increase or lower the basic rate of income tax by three pence in the pound — a power never used. But what has long been clear is that simply ignoring the political climate and insisting, as he has been doing, that there is no need for the Parliament to evolve any further was achieving nothing; obduracy was simply playing into the hands of the SNP, and, with another by-election looming in Fife, and every poll showing that the Scots wanted more power for Holyrood, the argument for change was irresistible.

Later this year the Calman commission will report on the nature of that change, but now Mr Brown has grasped the nettle and announced that he is prepared to bow to pressure, and consider granting the Parliament increased powers, including possibly the right to impose its own tax regime.

This is a seismic change, and Mr Brown will be aware of all the dangers that go with it. Fiscal independence, as it is known, could easily play into the hands of the Nationalists, who will argue that it is simply the curtain raiser to full-blown independence, fulfilling Tam Dalyell’s dictum that devolution is “a motorway without exits” to independence and the break-up of the United Kingdom. That does not necessarily follow.

For all the success Mr Salmond has enjoyed, the Scots show no more disposition to embrace independence than they did when the SNP took power. They want to see Holyrood’s powers developing and growing, but they do not want to see Scotland breaking away.

All that Mr Brown is doing is recognising the reality of what is happening in his own backyard. He will argue that giving the Parliament greater tax-raising powers is simply a way of increasing accountability, countering the SNP, and strengthening rather than weakening the Union. He will be all too aware, however, that by conceding the argument, he is in danger of losing control of the agenda in Scotland.

The Nationalists will make great play of the fact that it is they, not Labour, who have brought about this latest change, and Mr Salmond will claim that he has forced Mr Brown into a classic U-turn. If he is to wrest back the initiative, Mr Brown must move rapidly to explain precisely what extra powers he envisages the Parliament having and when these should be introduced. Just waiting for Calman to report may not now be enough. The devolution process is well under way. It will take boldness and imagination to control its direction.