A second referendum on Scottish independence might be necessary because clarity on some of the key issues would be impossible to achieve before the vote in September 2014, according to one of Alex Salmond's former key economic advisers.

Professor John Kay, an economist, who sat on the Scottish Government's economic advisory panel, was responding to the publication of the House of Lords Economic Affairs Committee report, which called for London and Edinburgh to set out their positions on key issues for the electorate. He made clear there would have to be a "long process of negotiation between Scotland and the rest of the UK to establish what independence actually meant".

Lord MacGregor, the committee chairman, said: "It's crucial the major implications are debated and understood by the voters well before the actual vote takes place.

"It's just not good enough to say so many of these issues can be solved after the vote by negotiations because so many of them are crucial to understanding on how to vote, such as currency, defence, the divisions of assets, what the division of debts is going to be and liabilities, negotiations with the EU because it's clear Scotland would have to negotiate its position into the EU. All of these issues need to be understood beforehand and that's why we are calling on both governments to indicate their red lines."

However, Mr Kay said: "You could have a vote on the principle of independence but then you have to have several years of negotiation and at the end of that time you might well have to ask the people of Scotland again – is this the package you wanted when you voted for independence?"