Independence could cost Scotland's economy £8 billion and jeopardise thousands of jobs, says Better Together leader and Edinburgh MP Alistair Darling.
Research by the pro-union campaign group into the so-called 'border-effect' shows that 247,000 Scottish jobs are dependent on cross-border trade with the rest of the UK.
But pro-independence campaigners have criticised the former chancellor's comments, claiming that the border Scots should be concerned about is the one between the UK and EU, Business for Scotland told IBTimesUK.
Spokesperson Paul Fletcher said: "With both Cameron and Miliband talking about having a referendum on staying in or leaving the EU, there's a real danger that Scotland will be separated from a single market of some 500 million people and that's a threat to business.
"What is clear is that Scotland needs to be part of the EU to take full advantage of the biggest single market in the world."
Darling has said pointed today to a recent research paper by economists at Stirling and Edinburgh Universities, which estimate that a new border would reduce Scottish economic output by 5.5%.
This would be equivalent to £8 billion a year, Darling claims.
Darling said: "The access to the single market of 63 million people in the UK rather than just 5 million in Scotland is good for Scottish jobs."
A poll for ITV Border recently showed support for a Yes vote in the Scottish Borders and Dumfries and Galloway council areas at 26%, with No at 61% and 13% undecided.