SNP ministers are being urged to release a secret batch of official documents about a second independence referendum.

The Scottish Tories demanded the material, produced by officials since 2016, be published after discovering its existence through a freedom of information (FoI) request.

The party said the files, some covering legal advice, showed a referendum was “never off the SNP’s table”, and have now lodged an appeal with Scotland’s information tsar.

However the depth and precise subject matter within the documents remains unclear.

The Tories made their FoI request on August 24, asking the Scottish Government for “details of any briefings by civil servants or documents provided to ministers in 2017 or 2018 on the topic of a second independence referendum”.

Ministers replied in late September, refusing to disclose material on the grounds it would prejudice the conduct of public affairs to reveal sensitive government material.

The government also said some material was exempt because it involved “legal advice”.

The only material released a single letter from then finance and constitution secretary Derek Mackay to a Holyrood committee about the government publishing an analysis of responses to its draft referendum bill consultation in June last year.

Following a challenge to the decision, the government has now revealed more information.

It said that, in addition to the previous letter, it could “confirm that 13 other documents were identified as being within scope of [the] request.

“These were provided from civil servants to Ministers as part of their normal duties supporting Ministers. We are unable to provide these items because of the exemptions listed in the response to your original request.”

The response acknowledged “a public interest in the topic of a second independence referendum and in disclosing information as part of open, transparent and accountable government, and to inform public debate”.

However it add that “after taking account of all of the circumstances of the case, there is a greater public interest in ensuring high-quality policy and decision-making by government, and this is achieved through allowing ministers and officials the private space they need to consider all available options and to debate those rigorously and to fully understand their possible implications, before a final position is reached.”

The Scottish Conservatives have now submitted an appeal to Daren Fitzhenry, the Scottish Information Commissioner.

Tory chief whip Maurice Golden said: “These secret briefings prove that the prospect of a second independence referendum is never off the SNP’s table.

“The question was settled in 2014. Yet, in the years afterwards, civil servants were still churning out briefings for ministers so they could continue banging the drum for separation in front of the cameras. That is irresponsible government.

“Instead, the nationalists should have been putting all their efforts into improving schools, hospitals, infrastructure and the economy.

“The SNP government clearly deemed it necessary for taxpayer-funded civil servants to produce these highly-political briefings.

“As such, they should now be published, as part of the SNP’s so-called commitment to open and transparent government.”

Pamela Nash, chief executive of the anti-independence group Scotland in Union, said: “This damning revelation proves that the SNP’s only priority is a divisive and unnecessary second independence referendum.

“It is staggering that ministers ordered taxpayer-funded civil servants to devote so much of their time to something that poll after poll has shown that Scotland doesn’t want.

“It’s little wonder that our economy is being neglected and public services are suffering when we have a Nationalist government that is so obsessed with constitutional division.”

The Scottish Government said the Tories were hypocritical, highlighting Ruth Davidson said last year that the SNP’s pursuit of independence was a “perfectly honourable position”.

A spokesperson for Constitutional Relations Secretary Michael Russell: “The stench of hypocrisy from the Tories is overpowering - not only have they blocked the release of official papers on the first independence referendum, they have also slapped gagging orders on business and charity leaders to prevent them talking about Brexit or welfare cuts.

“The Scottish Government has every right to discuss independence - something Ruth Davidson has described as ‘perfectly honourable’.

“And more and more people across Scotland are looking positively at the opportunities of independence as the Tories’ shambolic Brexit goes from bad to worse – quite simply, the Tories think they can do whatever they want to Scotland and get away with it.”