A court challenge against the Scottish independence referendum "is likely to be successful" if it does not have the legal backing of Westminster, the UK`s top law adviser in Scotland will say on Friday.
Advocate General for Scotland Lord Wallace will set out the UK Government's conclusions on the competence of the Scottish Parliament to legislate for a referendum on independence during a lecture at Glasgow University.
He will say that the Scottish Parliament "does not have power to legislate for a referendum on independence".
Speaking in advance of the lecture, Lord Wallace said: "The Scotland Act 1998 is clear: the Scottish Parliament cannot legislate on matters reserved to the UK Parliament."
He added that as the Union is a reserved matter, any Act of the Scottish Parliament that relates to the Union is "not law".
Lord Wallace, a former Deputy First Minister of Scotland and Scottish Liberal Democrat party leader, said: "A referendum must do something, and mean something, otherwise it is hardly a proper use of public money.
"It would not have a purpose in its own right and it is not merely an opinion poll — a referendum is, or at least purports to be, a form of political decision making, where a decision is best made directly by the public rather than by their representatives in Parliament.
"If, as the law currently stands, the Scottish Government were to proceed with this legislation, it would be open to challenge in the courts.
"It is the view of the UK Government that such a challenge is likely to be successful.
"It could be initiated in the Court of Session by any individual or organisation which could demonstrate sufficient interest to bring proceedings.
"The challenge could invalidate the outcome of the referendum, it would cause delay, and it could stop any referendum from happening in the first place."
A spokesman for Scotland's First Minister, Alex Salmond, 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.
"The Westminster parties should have a care, for their own sake — they have badly misjudged this issue, and badly misjudged the mood of people in Scotland.
"The sabre rattling of the Westminster parties — ganging up with each other, and trying to bounce Scotland into an early referendum and dictate the terms — has seen support for independence already reach 40%."
