Plaid Cymru leader Adam Price

He claimed the close collaboration between Belgium, the Netherlands and Luxembourg had not undermined the distinctiveness of the three countries but had given them a bigger presence on the international stage.

Mr Price told a seminar staged by the Centre on Constitutional Change at Edinburgh University that British identity would remain a reality for many people even if there was independence.

"When Scotland becomes independent and Wales become independent along with England we will still all be part of Britain, still sharing many things."

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He quoted former SNP MSP Andrew Wilson who had urged the SNP to be relaxed about some people's sense of Britishness and former First Minister Alex Salmond who had talked of the "social union" continuing after independence.

And Mr Price argued the Benelux arrangements - which involves trade liberalisation, free movement and monetary policy co-ordination between the three countries - could be a good model.

"No-one would deny that Belgium, the Netherlands and Luxembourg are distinctive countries. By operating together they have grown their economies and grown their presence on the international stage.

"The example is there of what can be possible with a post-independence union in these islands between Scotland, Wales and England.