SCOTLAND’S information watchdog has found the Scottish Government guilty of breaching freedom of information law by keeping ‘black spider’ letters from Prince Charles secret.

Rosemary Agnew, the Information Commissioner, has ordered ministers to release private correspondence with the heir to the throne to the Sunday Herald. She has also criticised problems the government had accessing the royal letters.

Two tranches of the Prince’s letters - given an arachnid nickname because of his spidery handwriting – were released in response to freedom of information requests last year. They showed how Charles lobbied ministers and public agencies on a wide range of pet projects, including red squirrels, bees, historic buildings, organic food and homeopathy.

According to Agnew, three letters to the then First Minister, Alex Salmond, were kept secret by the government because it thought their publication “would substantially prejudice the Prince’s interests as the future monarch”. The timing of the correspondence, the government argued, would prompt damaging “speculation of a political nature”.

But Agnew has dismissed the government’s arguments. “She does not accept that the information could be characterised as confidential, while the submissions on harm to the Prince's future role as monarch appear to be speculative and wholly unrelated to the nature of the information withheld,” said her decision notice.

The commissioner concluded that ministers “failed to comply” with environmental information regulations and were “not entitled” to withhold the Prince’s letters. She ordered them to be released by May 16.

Agnew found it “somewhat surprising” that the only way the government could locate and retrieve the Prince’s letters was by trawling through all the records held in different policy areas. She expected ministers “to take reasonable steps to ensure that systems are designed recognising the need to search for, locate and retrieve information efficiently.”

Campaigners critical of the monarchy welcomed her decision. “I hope this prompts a more open and transparent attitude from the Scottish Government,” said Graham Smith, chief executive of the anti-royal group, Republic.

“It reflects badly on the government that it has resisted the release of these letters on such spurious grounds. If Prince Charles is interfering in politics, against the conventions of our democratic system, the people have a right to know.”

Smith called on Scottish ministers to release all the correspondence they have received from the Prince. “They should own up to any undue influence Charles has had on Scottish policy,” he urged.

The Scottish Green Party’s justice spokesman, John Finnie, said: “It is of paramount importance that communications between the heir to the British throne and the Scottish Government are transparent.”

It was important to question “the culture of deference and secrecy” around the royal family and their dealings with government, Finnie argued. Of course the future monarch has the right to hold opinions, he said.

“But the notion that an unelected head of state can on the one hand brief ministers yet still be protected from public scrutiny is both antiquated and anachronistic.”

A spokeswoman for Prince Charles said: “We have no comment to make on this at this stage.” In the past the Prince, who is known as the Duke of Rothesay in Scotland, has argued strongly that his correspondence should be kept confidential.

“The publication of private letters can only inhibit his ability to express the concerns and suggestions which have been put to him in the course of his travels and meetings,” his spokeswoman told the Sunday Herald last October.

“The Duke of Rothesay cares deeply about the United Kingdom, and tries to use his unique position to help others. He has devoted his working life to helping individuals and organisations, to make a difference for the better of this country and the world.”

A spokesman for the Scottish Government said: “We received a decision from the Scottish Information Commissioner on March 30 and are currently considering its terms.”