Plaid Cymru, meanwhile, accused Labour in Wales of having no coherent policy, pointing out while Mr Hain was warning of the destruction of the union, first minister Rhodri Morgan asserted independence would not be an election issue.
"Labour in Wales is in disarray over their campaign. It's about time Labour focused on the issues that matter," said Plaid's deputy assembly leader, Rhodri Glyn Thomas.
The Conservatives have also hit back at Labour for suggesting they back a break-up of the union. On Sunday David Cameron accused the chancellor of using the "fear of the economic consequences of going it alone" to intimidate Scots voters.
Shadow Scotland secretary David Mundell has tabled an early day motion celebrating the 300th anniversary of the Act of Union, which was ratified by the Scottish parliament on January 16th 1707, as proof of his party's support.
"My motion sets out clearly my party's position on the union, recognising that it has been one of the greatest political success stories of modern European history,"
By Peter Hitchens,
"This revolution, though well under way, is not complete. Still to come are the destruction or at least the serious diminishing of the monarchy, the reduction of the power of the House of Commons, the transformation of the practice of law, the end of the pre-eminence of privileged institutions like the universities of Oxford and Cambridge, the disestablishment of the Church of England and the dissolution of the 1707 union of England with Scotland." But in the view of some historians, Scotland has benefited hugely from the Union , whatever the future holds.
Professor Whatley argues the Union cost Scotland its parliament and its sovereignty, but it brought big gains.
These included access to new markets, the protection of the Royal Navy at a time when the Scottish navy comprised three vessels, the Protestant succession at a time when religion mattered greatly, and the preservation of Scotland 's unique legal and education systems.
It also brought compensation - about £400,000 in money of the time - for Scotland 's disastrous Darien colonial experiment.
The Union was agreed despite strong public opposition in Scotland and the fear of lost identify had the paradoxical effect of prompting a new interest in Scottish culture.
"When you are in Scotland , you are in a different country to England ," said Professor Whatley.
"There are differences and in part, this reflects the fact that not all was lost in 1707."
He said: " Scotland certainly gained, but that loss of sovereignty and independence of action mattered a great deal.
"But I think in the world of the late 17th century, early 18th century and throughout the 19th century, this was the best option for the Scots.