Gordon Brown is stepping up his role in the independence campaign, after burying the hatchet with his former Downing Street neighbour Alistair Darling.

The former Prime Minister has asked the leader of the Better Together campaign, Mr Darling, to commission analysis to stress-test the Scottish Government's key proposal of a currency union between an independent Scotland and England.

The tests would be along the lines of Mr Brown's "five economic tests" for Britain joining the eurozone posed when Labour came to power in 1997.

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The tests, which were never met, asked: could there be sustainable convergence between Britain and the eurozone; was there sufficient flexibility to cope with economic change; what would be the effect be on UK investment; what would be the impact on the financial services sector; and was it good for jobs, stability and growth?

Danny Alexander, the Chief Secretary to the Treasury, said there was "no clear economic rationale" for a currency union between the UK and an independent Scotland.

The LibDem minister said that the so-called "fiscal gap" in an independent Scotland, identified by the Institute for Fiscal Studies and put at 1.9% of national income compared to 0.8% for the UK, would mean a break-away Scotland would need to grow at almost 2% a year more than the UK for the next 50 years in order to eradicate the gap.

"Over the past 50 years Scotland and the rest of the UK have grown, on average, at an almost identical 2% per year. So it would be necessary to double this rate," explained Mr Alexander.

"This would be at the same time an independent Scotland's economy would be facing greater pressure from the decline in North Sea production and an ageing population.

"No European country has managed to grow at the required rate of 4% over the past 50 years."

Labour sources have made clear the ex-PM will be a major presence in the pro-UK campaign with one making clear he will up his contributions significantly as we near September 18.

Another said Mr Brown, much more highly regarded north of the Border than south, would bring "poetry" and "emotion" to the debate.

l Mr Brown has claimed £1012 over two months last year for the rent of his Kirkcaldy and Cowdenbeath constituency office, the latest audit of MPs expenses has revealed.