Fresh blow to Salmond plans for pound and pensions

Leading academics also said the ­overall cost of the state pension would rise because of Scotland's higher proportion of OAPs, while public services would have to be cut, or taxes increased, to plug a hole in the country's finances.

Better Together, the pro-UK campaign, said the series of academic papers highlighted the "enormous challenges" Scotland would face if it left the UK. However, Finance Secretary John Swinney said the reports showed an independent Scotland would start out with the "firm foundation" of lower debt than the rest of the UK.

The respected NIESR published six academic papers on Scottish independence alongside its regular annual forecast for the UK economy.

In a new assessment of the Scottish Government's proposal to share the pound within a formal monetary union with the UK, Angus Armstrong and Monique Ebell of NIESR said a currency deal would be difficult to negotiate and would leave Scotland with limited control over how much it could tax, spend and borrow.

If a monetary union could be agreed, they warned it would be vulnerable to speculative attack from foreign exchange traders if the money markets sensed Scotland, able only to control its high levels of debt by imposing unpalatable spending cuts or tax rises, was about to pull out.

Dr Armstrong said the risk of the currency union crashing was so serious the UK Government should spell out immediately whether it was willing to strike a deal in principle.

He said: "You want this, one way or another, settled. We need to have a discussion about this. And the UK is able to have this discussion, it could say yes or no to a formal union.

"Having this degree of uncertainty over this most important issue, which could be clarified - either one way or the other, it could be clarified - I don't think it's helping people."

Chancellor George Osborne has said it is unlikely the UK would sign up to a currency union.

Mark Carney, Governor of Bank of England, has warned such a deal, if agreed, would place considerable constraints on an independent Scotland's fiscal policy.

In a separate paper, Professor David Bell, of Stirling University, said state pensions would be cheaper to fund on an individual basis because Scots died younger than the UK average.

However, overall costs to the taxpayer would rise because Scotland's ageing population would result in the country having a higher proportion of OAPs.

He warned the Scottish Government's pledge to delay a planned rise in the state pension age to 67 would cost taxpayers an extra £550 million per year.

In a third paper, Rowena Crawford and Gemma Tetlow of the Institute for Fiscal Studies said an independent Scotland faced a bigger black hole in its public finances because of its ageing population and declining oil reserves.

Assessing the SNP's White Paper on independence, they argued: "The specific policies proposed contained greater giveaways than they did take-aways. Ultimately this balance would need to be addressed."

Meanwhile, John McLaren and Jo Armstrong, of Glasgow University's CPPR think-tank, suggested claims an independent Scotland would be the sixth richest country, per person, in the world were misleading because of the high level of foreign ownership in the oil, energy and drinks industries.

Blair McDougall, campaign director of Better Together, said: "Expert after expert is lining up to point out the enormous challenges we would face if we lose the financial back-up of being part of one of the biggest economies in the world."

Mr Swinney said: "On any calculation an independent Scotland will have lower levels of debt than the rest of the UK and the firm foundations we need to build a stronger economy.

"It is only with the full powers of independence that we will be able to properly use all the economic levers other countries in Europe take for granted and grow the working population, increase productivity, boost exports and innovation and reindustrialise Scotland's economy."