FIRST Minister Alex Salmond would have "walked" the independence debate had the euro been a runaway success, a leading historian has said.

Princeton academic Professor Linda Colley, the presenter of a major BBC Radio 4 series on the historic forces that have united and divided the UK, also suggested that the future shape of the European Union could not be taken for granted, whether Scotland becomes independent or not.

Prof Colley said that she believed that the issues that afflicted the eurozone after the 2008 economic crash had affected the SNP's independence campaign.

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"This is just my off-the- cuff remark but it does seem to me the case that if the European Union was doing absolutely wonderfully well at the moment, and the euro was forging ahead on all cylinders, then I think Alex Salmond would have walked it.

"He might still do so. But I think the problems of the EU at the moment have also created a certain amount of difficulty for Alex Salmond.

"He can't say 'look at these forging ahead economies in continental Europe'."

Scotland's continued relationship with the EU has emerged as a major issue in the independence debate. Both sides accuse each other of leading Scots down a path that could see them, if only temporarily, outside the EU.

Prof Colley, whose series - Acts of Union and Disunion - also looks at the UK's relationship with Europe, warned it would be wrong to make assumptions about the future shape of the EU.

There were "serious issues of governance" affecting the EU, she said, including the fact that it introduced the euro too quickly.

"It took over 100 years for the US to establish itself as a unified polity, despite having a federal system, despite having a certain amount of ideological coherence - at least among its white population - and the US almost came apart during the 1860s.

"And here we are trying to collate a Union of different European countries, each with their own traditions and divisions, and a history of fighting each other, and you think that after 50 years you can give them a common currency and its all going to work?"

But Prof Colley suggested that an independent Scotland and the remaining UK could eventually work together in European Parliament, if there was felt to be an "overlap of interest".

The academic also said that if Scots reject independence in September's referendum she saw the merit in a federalised system within the UK.

"There are lots of deep regional divides within England not just north and south, Cornwall, the Mid-lands. An English Parliament that recognised regions - that might be useful".

A more explicitly federal structure within the UK "is probably the way to go", she added. This could see Westminster become a 'supranational' body that looks after issues such as climate control, foreign policy, migration and coastal erosion. "Whether there would ever be appetite for such a change I don't know. I think it should not be entirely ruled out as a possibility," she said

She insisted the idea should not be written off because of arguments that different parts of the UK are just too wildly different in size for it to work.

She pointed to the differences between US states such as Texas and Rhode Island, adding: "They seem to cope."

As well as having "enormous" other benefits, she said that federalism was a "way of organising diversity in such a way as to avoid complete separation".

Over the longer term she said that those advocating the pro-Union case had to constantly "refurbish" the idea. "Better Together, what does 'together' mean?" she said. "The lack of a persistently revised vision of this state nation, 'What does the UK mean? What is it for? You have got to get some kind of overall vision."

SNP MSP Clare Adamson said: "We welcome Professor Colley's engagement with the referendum, and are extremely confident of achieving a Yes vote in September.

"One significant issue is that Westminster is ­operating to a right-wing Ukip agenda that is ­threatening to drag Scotland out of the EU and single European market against our will, which only a Yes vote can protect ­Scotland from."