JOHN Swinney has said George Osborne was “wrong” to publish a letter from his top civil servant during the independence referendum campaign which warned about a currency union between the UK and an independent Scotland.

The deputy first minister hit out at the former chancellor over putting the document by Sir Nicholas Macpherson into the public domain.

The coalition government took the unusual step of publishing the official’s two-page argument against a sterling zone in February 2014.

The letter warned he would advise strongly against such an arrangement saying it would be fraught with difficulty. However, it triggered accusations that Macpherson was acting on the instructions of his boss when it was produced alongside the chancellor’s announcement formally ruling out a currency deal.

Attention was drawn to the episode yesterday during a SNP conference fringe meeting on the impartiality of the civil service.

Swinney defended Macpherson for writing the letter but criticised the then chancellor for publishing it, saying it put the civil servant in the public spotlight.

“I think the letter that Sir Nick Macpherson wrote to the chancellor was perfectly appropriate,” he said.

“What I thought what was inappropriate was the publication of that letter by the chancellor.

“Because what that did, and I don’t know this as I have never had a conversation with Sir Nicholas Macpherson about this point, but that put him into the public spotlight as opposed to the chancellor saying: ‘My view is ... ’

“And the chancellor is perfectly entitled to say that but what I didn’t think he was entitled to do is publish a letter which Macpherson had properly made available to the chancellor in his professional judgement.”

Dave Penman, general secretary of the FDA, the trade union for senior and middle ranking civil servants, was also highly critical of Osborne.

“I think George Osborne decided to publish that because he thought he added weight to his argument and I thought that exposed the civil service in a way, given how difficult that situation was in the referendum for civil servants serving two governments with such diametrically opposed policies, I thought was indefensible,” he said.

Pressed that the publication of the letter wasn’t against Macpherson’s will, Swinney said such permission was irrelevant.

Earlier in the discussion Swinney said the SNP could trust the impartiality of civil servants and pointed to the warm welcome a former head of the Scottish Government civil service had given him after the SNP were first elected to government in 2007 and just after he had served the Labour/LibDem coalition in Edinburgh.

Meanwhile Penman told of death threats made against some senior civil servants and the need for them to be given increased security because of public discourse in the current Brexit process.