The Scottish Government risks flouting a "fundamental principle of democracy" if it stages an "unlawful" vote on independence, the Advocate General for Scotland said today.
Lord Wallace of Tankerness QC restated the UK Government's view that the "Scottish Parliament has no power to deliver a referendum on independence".
First Minister Alex Salmond has insisted his plans to hold a vote on the country's constitutional future in the autumn of 2014 are legal.
That stance is at odds with the Tory-Liberal Democrat coalition at Westminster, with ministers there arguing the legislation that set up the Scottish Parliament did not give it power over constitutional issues.
Last week Scottish Secretary Michael Moore offered to temporarily extend Holyrood's powers so it could hold an independence ballot - if certain conditions are met.
Mr Moore also asked Mr Salmond for a meeting in Edinburgh this Thursday - but this was rejected by the Scottish Government as it would come before the launch of its consultation paper on its referendum plans.
Lord Wallace said it would "make a great deal of sense" for the two men to meet to "start clearing the path to a legal, fair and decisive referendum for the Scottish people".
He said: "There is no reason to delay these talks while we await the Scottish Government's own consultation paper because there is nothing that could conceivably be in that document that will alter the legal position we face."
Lord Wallace, the UK Government's principal legal adviser on Scots law, added: "Let me be clear. The Government wishes the referendum to take place on a sound legal basis, thus avoiding court challenge.
"The Government has therefore proposed a solution that involves devolving power to the Scottish Parliament that would avoid a referendum Bill being dragged through the courts."
Lord Wallace, a former Scottish Liberal Democrat leader and ex-deputy first minister, said that if the SNP administration were to "proceed with a referendum that is outside of its legal powers would be to act contrary to the rule of law".
He continued: "This is not a mere legal technicality as some commentators have suggested. Government according to law is a fundamental principle of democracy. To flout this principle would be a very worrying step for a democratically-elected Government to take."
He also warned: "Scottish Parliament legislation paving the way for an unlawful referendum would be open to challenge in the courts.
"It is important to note that a successful challenge in the courts could not only invalidate the outcome of the referendum - it could stop the referendum happening in the first place."
Lord Wallace continued: "The UK Government acknowledges the political mandate achieved by the SNP at the May 2011 election. It is precisely because of this political mandate that the UK Government is offering means to ensure that the Scottish Parliament has the power to deliver a referendum that is fair, legal and decisive.
"The Section 30 order included in the Government's consultation paper would achieve this, and we are consulting in order to take the views of others on the issue."