Scotland’s chief civil servant has been urged to seek ministerial direction on whether the Scottish Government can continue to spend public funds on independence referendum plans.

It follows the Supreme Court ruling that the Scottish Parliament cannot legally call a referendum without Westminster approval.

Donald Cameron, constitution spokesman for the Scottish Tories, has written to John-Paul Marks, permanent secretary to the Scottish Government, to assess whether the forecasted spend of £20 million on referendum preparation is “lawful”.

He accused the Scottish Government of using public funds for “party propaganda”.

Figures obtained by the Tories on Sunday revealed more than £1.5 million of taxpayer cash would be spent on civil servant salaries who are tasked to work on the independence brief.

But in his letter to Mr Marks, the Tory MSPs wrote: “Given that the Supreme Court ruled that ‘the Scottish Parliament does not have the power to legislate for a referendum on Scottish independence’, there is 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.

“I am therefore seeking urgent clarification on whether this remains the position of the Scottish Government, and also to request that you seek a “ministerial direction” to settle this matter.”

After sending the letter to the senior civil servant, Mr Cameron said he struggled to see “any legal clarification for ministers to continue to lavish” the estimated £20 million.

He added: “That’s why I have written to the permanent secretary for clarification. I believe he has a legal duty to seek ‘ministerial direction’ on this because it looks as if the Scottish Government is allocating public money to something that is beyond its legislative competence.

“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 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.”

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 could 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.”