A leading judge acted unlawfully by authorising a secret court hearing in which he decided that Prince Philip’s will should be kept secret without notifying the media, an appeal court has heard.

On Wednesday the Guardian opened its legal case to overturn the decision that prevented media from attending the hearing, arguing that it was a serious interference with the principle of open justice.

Sir Andrew McFarlane, the president of the family division of the high court, ordered the sealing of Philip’s will for 90 years last September after a secret hearing in which he approved a confidential application from lawyers representing the royal family. The Queen’s husband died at the age of 99 in April 2021.

In a practice dating from 1911, high court judges have approved the closure of the wills of 33 members of the royal family after similarly secret court hearings and applications from the Windsors’ lawyers. The judiciary has never refused such a request from members of the Windsor family.

The rulings meant that these members of the royal family, some of them distant relatives, have been exempted from a law stipulating that the wills of British people are ordinarily open to being inspected.

At the start of the hearing, Caoilfhionn Gallagher QC, for the Guardian, said it was wrong that the media had not been notified about the hearing to close Philip’s will and were therefore unable to attend or making submissions in favour of open justice.

She said: “This was an entirely private, closed hearing, without access by accredited journalists or any other form of external scrutiny. An entirely private hearing such as this is the most serious interference with open justice.

“It is an exceptional step that requires exceptional justification. In this case, [McFarlane] decided to take such an exceptional step without even inviting or permitting members of the media to make submissions about whether such a procedure was fair or justified.”

The legal challenge is opposed by lawyers for the royal family and the attorney general, both of whom made confidential submissions last year to keep secret Philip’s will and exclude the media from the original hearing.

Last September McFarlane ruled that it was justified to exempt senior royals from the general rule requiring the publication of wills. “It is necessary to enhance the protection afforded to the private lives of this unique group of individuals, in order to protect the dignity and standing of the public role of the sovereign and other close members of her family,” he decided.

McFarlane had also decided that it was right that the attorney general had represented the public interest in the private hearing.

Lawyers representing the executors of Philip’s will and the attorney general argue that McFarlane acted properly, submitting that the media had no right to attend the hearing.

In a written submission, James Eadie QC, for the attorney general, said the context in which the application to seal the will was made was “exceptional”, adding: “Prince Philip was a senior member of the royal family as consort to H[er] M[ajesty] the Queen. His death had occurred only a few months earlier.”

He added that the practice of sealing wills such as Philip’s was part of a “consistent and longstanding” convention over more than a century.

He rejected the Guardian’s argument that McFarlane acted wrongly when he allowed the attorney general to represent the public interest. Eadie said: “The attorney has a proper, well-established, non-political role as the independent guardian of the public interest in the administration of justice.”

Jonathan Crow QC, for Farrers, Philip’s executors, described the challenge as “an utterly barren procedural appeal” and said the attorney general was “the only person entitled to come to court to address the public interest”. He said the media had no legally enforceable right to be heard in cases in which they were not parties.

“Accordingly it cannot be said that [McFarlane] made an error of law purely and simply because no media representatives were heard before a decision was taken to hear the matter in private,” he said.

The hearing, which is being heard by Sir Geoffrey Vos, master of the rolls, Dame Victoria Sharp, president of the Queen’s bench division, and Lady Justice King, continues.

On Monday the Guardian reported that the secrecy prevented the public from seeing how assets worth at least £187m at today’s prices, which were outlined in the 33 closed wills, were distributed.