The Scottish Government has been given a deadline to reveal whether it has received legal advice on the country’s status in the EU after independence.
Rosemary Agnew, the Scottish Information Commissioner, says the Government "failed" to comply with legislation and has given ministers until August 21 to confirm or deny they hold the information.
However, she stopped short of ordering the actual release of any advice.
It means the public will find out whether the SNP administration sought early information on a major policy for independence - automatic EU membership, without the need to reapply as a successor state after leaving the UK.
First Minister Alex Salmond has previously claimed that Scotland would automatically become a member state should voters choose independence in the 2014 referendum.
Opposition parties have speculated that legal advice may contradict the SNP position.
The commissioner's decision comes months after Labour MEP Catherine Stihler tried unsuccessfully to get information from ministers.
Ms Agnew's ruling states: "The question of whether independence will result in Scotland automatically remaining a member of the EU, or automatically being excluded and having to apply for membership, could have a bearing on how people vote in the referendum, depending on how they view the consequences of either of these outcomes."
She also accepted that any legal advice would concern the early stages of policy development and as such should remain private.
Ms Stihler said the pressure to reveal the contents is now overwhelming following the “landmark” judgement.
"People have a right to know whether an independent Scotland would be part of the EU and on what terms, but the SNP want to keep it secret," she said
"They actually claimed it was contrary to the public interest for us to know, even though their assertions are questioned by many contemporary experts.
"By refusing to confirm or deny, Alex Salmond effectively took out a super-injunction against the people of Scotland.
"Now, the Information Commissioner has ordered him to own up. She has ruled that approach is in breach of the law, which is a groundbreaking and welcome decision.
"The pressure on the SNP to release this advice is now overwhelming. They should just be straight with us: tell us whether the advice exists, and if it does, release it now."
The Scottish Government said it would appeal the decision, saying papers would be lodged by the August deadline.
A spokesman said: "We were surprised by the Information Commissioner's decision. It is the longstanding and usual practice of the Scottish Government to neither confirm or deny the existence or the content of legal advice.
"The approach we have taken on this issue is consistent with the UK Government position in a similar case they dealt with under equivalent legislation. We therefore intend to appeal and contest the decision."
Experts have put forward three likely options for post-independence Scotland and what would remain of the UK.
These include both parts continuing in the EU after separation, Scotland being forced to re-apply for membership while the rest of the UK continues, and "dissolution", where both parts would be forced to re-apply.
While political parties focus on the scenario they prefer, voters have not been given a definitive answer.
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