IN a major speech in London last month, First Minister Alex Salmond found a striking phrase to sum up an argument that is at the heart of his case for independence.

He described the UK capital as the "dark star of the economy". London, he said, was "inexorably sucking in resources, people and energy" from the rest of the UK to the detriment of its other nations and regions. An independent Scotland, he promised, would be "a northern light" to rebalance the economy of the British Isles.

The idea that Scotland (and everywhere else in Britain) is losing out because of the gravitational pull of London, and Westminster policies conceived mainly for its benefit, has become a key theme for the SNP. It is not only a Nationalist view. Vince Cable, the LibDem Business Secretary, last year likened the city to "a great suction machine draining the life out of the rest of the country" and today Ed Miliband will add his concerns. "The country of the industrial revolution," he will say, "has become a country which builds its prosperity far too much in one city: London."

Speaking in Birmingham, the Labour leader will set out plans to reverse the centralisation of economic power in London. His answer is not independence for Scotland, of course, but a significant devolution of economic development powers to the UK's other great cities. He will take care to name Glasgow along with the likes of Manchester, Liverpool and Leeds when he lists the places he believes would benefit from a transfer of powers over transport, housing development and job creation worth billions of pounds. The proposal is likely to be greeted warmly in the proud Victorian city halls of places north of Watford; places, as it happens, with many Labour and not so many Conservative councillors. Mr Miliband hopes it will satisfy a growing appetite for devolution across the UK which, south of the Border, was only temporarily suppressed by John Prescott's clumsy and costly plans for regional assemblies a decade ago.

In the north-east of England, for example, where the then-deputy prime minister's proposals were humiliatingly rejected, there are growing fears the region will be squeezed by either an independent or more powerful Scotland following September's referendum.

North of the Border, Mr Miliband's cities plan will become a major part of Labour's devolution "offer" in the event of a No vote in September's referendum. Scotland's cities, Labour will pledge, will be given the power to thrive as part of the UK. Johann Lamont, the Scottish Labour leader, has backed proposals to give legal recognition to the status of councils in a move designed to protect them from Holyrood's centralising tendencies.

Mr Miliband's plan throws up some potential complications, given the patchwork of powers affecting councils that are devolved to Holyrood and reserved to Westminster. But the message is clear: cities across the UK should enjoy greater autonomy and responsibility for their own success. The reaction from Glasgow and Edinburgh will be interesting, to say the least.