Alistair Darling has reiterated his call for the SNP to set out what an independent Scotland's economy would look like in a message to business leaders.

The head of the pro-union Better Together campaign warns in an article for the Institute of Directors, which will be sent to its members, that not enough information has been given to voters.

The former Chancellor said businesses across Scotland were concerned about potential trade barriers, the currency of a separate Scotland and future membership of the European Union in the event of a Yes vote in the referendum.

But the Scottish Government hit back, saying the threat to Scotland's EU membership came not from next year's independence referendum but from the in-out EU vote David Cameron has pledged to hold in 2017.

Mr Darling asks: "Would we have to join the Euro? Would we keep the vital opt-outs? Would we lose our rebate?

"Would we have to join the EU immigration system, which would mean that our immigration protocols would be different from those operated by our near neighbours south of the Border?"

"These are important questions that have yet to be answered by the Nationalists.

"I don't ask them to score political points. I do so because the answers are of fundamental importance to the future prosperity of Scotland, including our businesses."

"We need to know the answers to these and other legitimate questions. We cannot be asked to take the biggest decision in 300 years on the basis of the nationalists telling us not to worry, that everything is going to be all right on the night."

A Scottish Government spokesman said: "An independent Scotland will keep the pound.

"We will also remain part of the EU - and it is Westminster's proposed in-out referendum on EU membership that poses the only threat to ­Scotland's continued membership, with all the potential consequences for investment, jobs and the economy which that involves."