The Scottish Conservatives have demanded the civil service seek clarity over the legality of SNP ministers using public money on their campaign for independence. 

Donald Cameron, the party's constitution spokesman has written to Permanent Secretary, John Paul Marks, questioning the £20m ringfenced in next year's budget for a new vote.

The row comes after the Supreme Court ruled that Holyrood cannot hold an independence referendum without Westminster’s consent.

The Tory MSP said that meant there was "now significant uncertainty as to whether the forecast use of £20 million of taxpayer money on an issue that is not within the devolved competence of the Scottish Parliament is lawful."

He said Mr Parks should seek “ministerial direction” - a formal instruction telling him to proceed with the spending proposal, regardless of any objection from civil servants.

Meanwhile, the pro-union Scottish Business UK (SBUK) group has threatened legal action against Ms Sturgeon over the taxpayers' cash row.

They said the SNP were behaving like the government of "a South American banana republic”.

However, that argument was given short shrift by a number of legal experts. Aileen McHarg, Professor of Public Law and Human Rights at Durham University, described it as "a ludicrous position to adopt." 

In the wake of last week's court verdict, the SNP leader confirmed that she would now fight the next general election - not expected until 2024 - as a de facto referendum. 

A briefing note sent to SNP politicians also confirmed that the Scottish Government “will continue to make the case for independence and for the Scottish people to have their say” and that they will produce more documents “published in the coming months.” 

It goes on to say that the papers in the Building a New Scotland series still to come "will cover matters including pensions and social security, EU membership, energy, and defence and security”.

According to a recent response to a Freedom of Information request, there are currently 25 officials working on the independence prospectus and related work.

In his letter to the chief mandarin, Mr Cameron - an advocate before he entered parliament - said in his view, Mr Parks had "a legal duty to seek a 'ministerial direction' in light of the fact that this spending proposal is beyond the power of the First Minister (and indeed of any minister in the Scottish Government.)"

He said it looked as if the Scottish Government "is allocating public money to something beyond its legislative competence."

In a statement, Mr Cameron added: “Nicola Sturgeon said her next step, after the Supreme Court’s judgment, was to try to turn the next General Election into a ‘de facto referendum’.

“But that’s SNP strategy as a party – not a Scottish Government policy. There is no excuse for impartial civil servants to be deployed on party propaganda in this way, and no reason for taxpayers’ money to be wasted on it.

“In the midst of a global cost-of-living crisis and huge cuts being imposed by the SNP on key public services, it’s outrageous for ministers to spend money pushing for a referendum that most Scots don’t want. Now there are question marks over the legality of this approach too.”

Robert Kilgour, chairman of SBUK, told the Daily Mail: “Nicola Sturgeon is using public taxpayers' money and civil servants for party purposes. We're seeking legal advice on the options in front of us. Enough is enough."

His intervention follows on from a question on the legality in the House of Lords from Labour peer George Foulkes. He also plans on raising it again next week. 

Over the weekend, Professor McHarg rubbished the proposal. 

“It only requires a moment's thought to know that it's a ludicrous position to adopt,” she tweeted.

“Holyrood legislation is (theoretically) at risk of challenge indefinitely. In Salvesen v Riddell [2003 Holyrood legislation to protect tenants in limited partnerships overturned by the Supreme Court in 2013] a provision was struck down 10 years after enactment.

"Should all the money spent on enacting it have retrospectively been declared unlawful? 

“How would you separate out the sum involved from the rest of the statute in question? 

“Would any expenditure on unsuccessfully defending the legislation in court also have been caught? 

“And what of the fact that the Scotland Act can be amended (by a s30 order or primary legislation)? 

“Would it be unlawful for ministers and civil servants to spend any time thinking about new powers until they were actually in force?

“All in all, this is a cheap line advanced by people who obviously don't expect that they will actually be in a position where they would have to try to govern under these conditions any time soon.  

“Deeply unserious politicians.”

Taking to Twitter, Adam Tomkins, a professor in the University of Glasgow's law school, and a former Tory MSP, said it was "not unlawful for the Scottish Government to pursue independence as a policy goal, nor to spend public money in that pursuit, nor to instruct civil servants to work on indy-related projects."

He added: "All the UK Supreme Court decided is that the Scottish Parliament has no legislative competence to enact law triggering an indyref. 'Lawmaking' and 'authorising public expenditure' are not the same as each other and are treated quite differently in law. Always have been."

A Scottish Government spokesperson said: "In light of majority support within the Scottish Parliament for an independence referendum, Scottish ministers remain ready to engage with the UK Government at any point on delivering that referendum.

"The Scottish Government will also continue to set out through the Building a New Scotland prospectus series what can be done with the full powers of independence, reflecting its Programme for Government commitments.

"It is the role of the civil service to support the elected government of the day in developing and implementing its policies."