In an article for The Herald, Sir David Edward, a former European Court judge, said Scotland would be launching itself on an "unpredictable sea of uncertainty" if it left the UK.
He said that, while the SNP suggested "everything we value in the Union will remain the same", Scotland and the rest of the UK would pursue their own interests, throwing present arrangements into doubt.
Sir David, an emeritus professor at Edinburgh University, also said he was disturbed by claims that only independence would allow Scots to regain a sense of identity and realise their potential.
A spokesman for the First Minister stressed the SNP's case for independence was not based on Scottish identity.
Sir David is one of the most cited experts in the constitutional debate. His views on an independent Scotland's membership of the European Union have been used regularly to support the SNP's claim that joining the bloc would be seamless and straightforward.
Two years ago he challenged comments by the outgoing president of the European Commission, Jose Manuel Barroso, who suggested an independent Scotland would have to reapply for EU membership.
Sir David argued that membership could be negotiated before Scotland formally became independent and existing EU treaties amended to accommodate the new state. Since then, he said he had found himself described as "the poster child of the Yes campaign".
Sir David says he stands by his assessment that an independent Scotland would not be automatically excluded from the EU and accepts the First Minister is entitled to cite his work "within limits".
But he adds: "This is as far as I can go."
He says the outcome of negotiations "cannot be certain" and warns that, although an acceptable deal could be achieved, it would not be reached by the Scottish Government's proposed independence date of March 2016.
Sir David concludes: "Siren voices urge us to vote Yes or No on the basis of unprovable (and often improbable) predictions that we will personally be better off or worse off.
"It is said that the Union of 1707 was purchased by English gold. As Dr Johnson said: 'That is no defence: it only makes you worse'. Anyhow, I fear that, this time, the gold is at the end of the rainbow."
In a keynote speech in Bruges in April, the First Minister praised Sir David as "one of the true architects of the European Union" and quoted the legal expert as saying it would be absurd for Scotland to have to leave the EU only to reapply.
Sir David disagreed with European Council President Herman Van Rompuy's use of moral arguments against separation. He argued instead that, although he disagreed with it, a desire for independence was consistent with the founding values of the EU.
A spokesman for Mr Salmond said: "Sir David is hugely respected in his field, and is certainly entitled to his views on Scotland's future. But the fact he is opposed to independence simply lends even more weight and authority to his expert views on Scotland's place in Europe, where he has made clear Scotland cannot be deprived of the rights and status we already enjoy within the EU.
"In terms of the wider case for independence, it is not about regaining Scotland's identity, which is already secure. It is about democracy, about always getting the governments we vote for and ensuring a wealthy country becomes a more prosperous and fairer society."