The papers show ministers were told they could work on policies preparing for a vote, and to test a question with the Electoral Commission.

But there was no reference as to whether Holyrood would have the power to legislate for indyref2 without the UK government’s backing.

The Scottish government had initially refused to publish the advice.

It was ordered to do so by the country’s information commissioner, who said there was “exceptional” public interest in seeing the advice.

The government had argued that making the advice public would breach legal professional privilege.

It has now published some of the advice on its website, but said the commissioner had ruled that it was entitled to withhold some of the details that had been asked for by the Scotsman newspaper under freedom of information laws.

The two-page document said the advice from the Scottish government legal directorate in January last year was that “there is a legal basis for Ministers to ask (the Electoral Commission) to undertake question testing”.

A month later, the government was told that law officers had previously confirmed that “ministers can lawfully undertake policy development work preparing proposals for independence and in calling for a transfer of power”.

However, there is no mention of any advice the government received on whether or not it has the power to hold a referendum even if the UK government does not grant formal consent.

Constitutional experts have been split over the question, and there has been speculation that any attempt by First Minister Nicola Sturgeon to press on with her plan to hold a referendum next year could result in a court battle between the governments in London and Edinburgh.

That is because the UK government has shown no sign that it would be willing to give the formal consent that ensured the legality of the 2014 independence referendum, in which voters backed remaining in the UK by 55% to 45%.

Opposition parties have argued that it would be a waste of public money for the government to press on with its plan for another referendum if it has already been told by its own lawyers that it does not have the powers to hold one.

When questioned about whether she would be asking Boris Johnson for a transfer of power, Ms Sturgeon said: “I’ll set out more about this, in substance and process, in the period ahead. I have a mandate for another referendum won fairly and squarely in an election last year and anyone who has any respect for democracy will understand the importance of honouring that mandate.”

There are two interesting things about this advice, but neither of them are included in the page-and-a-half of A4 published.

The first is the fact it was released at all. The government is fierce in its defence of the principle of legal counsel remaining private, and do not want anything to set a precedent that would change that.

The information commissioner ruled that this principle can, at times, be overruled by the public interest, and this in itself may have ramifications in future.

The second interesting thing is what isn’t in the document – which is to say, advice about anything particularly controversial.

Nobody had questioned the legality of either of the actions discussed – testing questions and developing policies – in fact, most had assumed the government was already doing them.

The really key legal question around a referendum is whether the Scottish Parliament could legislate for one, even without a transfer of power from Westminster.

Of that, there is no mention – either because ministers have managed to keep that advice private, or because they haven’t asked for it in the first place.

The Scotsman had originally asked for the legal advice to be released in January last year, and appealed to the information commissioner after the government refused to publish it.

The government had been given until 10 June to publish the legal advice by Information Commissioner Daren Fitzhenry, who said that keeping it secret would “actively harm accountability and scrutiny and would be counter to the public interest”.

He also said that publishing details of the legal advice would “significantly enhance public debate on this issue”.

Opposition parties said the information that had been published left the big question unanswered.

Scottish Conservative constitution spokesman Donald Cameron said it was “welcome that the SNP have finally been dragged into releasing this information that they tried to hide from the public”.

But he added: “It still leaves unanswered questions about how they plan to continue their push for a second divisive referendum.

“The murky secretive approach must end. The public deserve answers about what the SNP are planning.”

His view was echoed by Scottish Labour MSP Sarah Boyack, who said: “Another referendum is the SNP’s answer to every question under the sun, so the public shouldn’t be kept in the dark on the legality of it.

“The SNP have dragged this circus out for long enough – they need to come clean once and for all.”

Scottish Liberal Democrat leader Alex Cole-Hamilton said: “The Scottish government are at it. What the public want to know is whether the Scottish government has legal advice on holding a referendum without a Section 30 Order and by refusing to publish that they are mocking freedom of information legislation.”