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A Scottish Parliament bill to organise an independence referendum without the approval of Westminster would not be legal, the advocate general has warned.

Lord Wallace of Tankerness, a former deputy first minister, said such a vote could be challenged successfully in the courts.

He told 91�ȱ� Scotland Holyrood does not have the power to create legislation which relates to the constitution.

SNP ministers insist they could hold a vote without Westminster's permission.

Former Liberal Democrat MSP Lord Wallace was the deputy first minister in Scotland before leaving Holyrood and becoming the UK government's senior Scottish law adviser.

He has said that proceeding with a referendum outside of the Scottish Parliament's legal powers would be contrary to the Rule of Law and a worrying step for a democratically-elected government to take.

In a lecture in Glasgow on Friday, he argued that recent case law had widened the range of individuals who could mount a legal challenge.

Lord Wallace told 91�ȱ� Radio Scotland's Good Morning Scotland programme: "A referendum that would put in train negotiations, if it was successful, to change the union - the relationship of the union between Scotland and England - that, we believe, is not law because it exceeds the power given to the Scottish Parliament under the Scotland Act."

The advocate general, who has responsibility to scrutinise legislation passed at Holyrood, said the issue of whether the UK government would mount a legal challenge was hypothetical.

He added: "A case last year - Axa Insurance - did actually extend the people who might qualify to bring a case.

"It's not whether the UK government would bring a case, many, many individuals could bring a case and we don't want this issue to end up as a wrangle in the courts."

First Minister Alex Salmond has said his preferred date for the independence referendum is autumn 2014.

The coalition government last week launched a public consultation into the referendum.

'Entirely confident'

Mr Salmond will announce details of his government's consultation into the historic vote when he addresses the Scottish Parliament on Wednesday, 25 January.

The UK government has offered to give legal authority on holding a referendum to the SNP government if it asks a simple yes or no question.

Scotland's SNP government has said it does not need that authority to proceed.

A spokesman for the first minister said: "The UK government has published its opinion, not its legal advice - and the Scottish government have set out our position in a number of documents since May 2007.

"We are entirely confident about our legal basis to hold a consultative referendum - as confirmed by a range of experts.

"In any event, we have absolutely no objection to a Section 30 enhancement of Holyrood's powers to hold a legally binding referendum, but what we cannot possibly accept is Westminster dictating the terms of Scotland's referendum."

If Westminster and Holyrood fail to resolve the problem it could end up being ruled over by the Supreme Court, which sits in London.

After the Scottish National Party won an overall majority at the Scottish Parliament election in May last year, it pledged it would hold a referendum on the independence of Scotland towards the "latter part" of its five-year parliament.

Mr Salmond last week named autumn 2014 as his preferred time period. It is unclear at this stage if he will name the day when he makes his statement next week.