SNP MP Tommy Sheppard, the party’s constitutional affairs spokesperson, made a freedom of information request in June 2019 seeking information about polling since January 2018 on public perception on the union, including how much money had been spent on it.

The Cabinet Office did not disclose the information, citing an exemption clause relating to the development of government policy.

The MP appealed to the Information Commissioner’s Office, arguing that the clause should not apply because the UK government had no stated intention to review or alter policy in respect of the union.

This week, the First-tier Tribunal (General Regulatory Chamber) Information Rights ruled that the information that had been requested about the polling "relates to the implementation of existing policy rather than to policy development" and that the Cabinet Office must disclose the information within 28 days.

Sheppard said: "This is a very welcome decision with significant consequences. The Cabinet Office has been hiding this data claiming that they were developing policy. I have been clear from the start that this wasn't the case – maintaining the union is clearly an ongoing policy of the UK government – and the tribunal has come to the same conclusion.

"We have the right to know how much of taxpayers’ money has been spent on it, and what the polling says. If it’s paid for by the public, the results must be available to the public, otherwise the UK government is using taxpayers’ money to further their own political ends.

"Not only do we need to see the data I originally asked for, I have today made a further FOI request for data that has been gathered in the past two years."

A UK government spokesperson said: "The UK government regularly commissions research in different parts of the UK to understand public attitudes and behaviours to inform our campaigns and policies. This enables us to deliver strong national and cross-government communications campaigns.

"When considering making research public, we balance our commitment to transparency with the need for ministers to make decisions on the development of policy based on the data."