The Scottish Government breached transparency laws by not releasing analysis on how long it would take for an independent Scotland to join the EU.
The Scottish Information Commissioner has ordered ministers to release some information it kept secret in response to a Freedom of Information (FOI) request.
The request asked what analysis had been undertaken by the Scottish Government on the timescale of an independent Scotland joining the European Union.
But ministers refused to release some of what it found, arguing it was outwith the scope of the request.
They feared their release could “inhibit substantially the free and frank exchange of views for deliberation”.
While commissioner Daren Fitzhenry accepted some of the analysis should remain secret he said other parts should be released.
“The public interest in voters knowing the details of analysis conducted to determine the likely timescales involved in an independent Scotland re-joining the EU is significant and cannot be underestimated,” he said in his ruling, which partially upheld the complaint against ministers in Edinburgh.
Among the documents the Scottish Government argued were out of the scope were those on the timescale of other nations joining the political bloc.
Mr Fitzhenry said there would be “no apparent reason for any such information to be held, unless the experience of those other nations was considered relevant to how long it might take Scotland to join the EU”.
The commissioner said some of the information withheld was “relatively mundane”, adding “it does not offer a frank and politically unacceptable view, rather it makes deductions which are unlikely to surprise the reasonably informed observer.”
He went on: “The Commissioner does not accept that its disclosure would, or would be likely to, prevent officials from providing free and frank views in the future.”
Mr Fitzhenry said it was in the public interest for some of the information previously withheld to be released.
He said: “Given the prominence and importance of the issue of an independent Scotland being able to re-join the EU, the Commissioner considers that, if the ministers held any advice on how long that process would be likely to take, then its disclosure would be very much in the public interest.
“The Commissioner takes the view that the length of time it would take to re-join the EU might well be a key factor for individuals in deciding how to cast their vote in any potential second referendum on Scottish independence.”
The Scottish Government has until August 17 to hand over the information that was previously withheld.
Ministers are entitled to appeal the decision.
Scottish Conservative MSP Donald Cameron said the ruling was “another example of the SNP’s secret Scotland”.
He said: “Far from being the most open and transparent government as Nicola Sturgeon promised, the SNP have become addicted to cover-up and secrecy.
“The verdict from the Information Commissioner is damning. It should not take such a ruling to force SNP ministers to show this information to the public.
“There is a rotten culture of secrecy at the heart of the SNP government with ministers all too happy to keep the public in the dark.
“In light of this ruling, SNP ministers must guarantee that these documents will now be freely available as quickly as possible.”
A Scottish Government spokesperson said: “Scottish ministers note this decision from the Scottish Information Commissioner and will now consider their response.”