In a sharply worded ruling, David Hamilton, the information commissioner, said the government had failed to justify withholding internal correspondence between ministers and special advisers on the high-profile case. He has ordered officials to review the request.

In June 2022 Nicola Sturgeon instructed Lord Advocate Dorothy Bain KC to ask the Supreme Court whether Holyrood had the power to stage a consultative referendum without Westminster’s consent. The court later ruled unanimously that it did not, as the matter is reserved.

An FOI request lodged the following month sought internal communications about the case, including messages sent via WhatsApp, Signal and SMS, and asked ministers to confirm whether such records existed.

The government initially rejected the request in August 2022 under section 12(1) of the Freedom of Information (Scotland) Act, which allows refusals if the cost of compliance exceeds £600. A review request was also rejected, with officials citing a projected cost of £1,624.50 and claiming more than 700 documents would require thousands of minutes to sift and redact.

The applicant accused ministers of inflating figures to delay disclosure and lodged a complaint with the commissioner.

During the investigation, officials produced three different sets of cost estimates ranging from £1,110 to £1,624.50, each exceeding the ceiling. The commissioner found these figures inconsistent, noting that one search suggested 700 documents were in scope while another identified 1,128.

He criticised the reliance on extrapolations and assumptions that almost every document would need redaction. He also highlighted the government’s refusal to provide screenshots of its searches – something it has done in other recent FOI cases.

A Scottish government spokesperson said: “Scotland has the most open and far-reaching freedom of information legislation in the UK and we remain committed to our obligations under the Act to ensure openness and transparency. The Scottish Information Commissioner’s decision has been received and we are considering its terms.”