The King made a surprise appearance at the High Court a day before his son the Duke of Sussex is to discover the outcome of his hacking claim against a tabloid newspaper publisher.

Charles, while touring the Royal Courts of Justice in London on Thursday afternoon to celebrate the work of the judiciary, walked in on students who had just participated in a mock trial in a courtroom.

The King joked “I’m so sorry to interrupt” before chatting to the young people from Ashcroft Technology College in Putney, south-west London.

He was shown round the building by the Lady Chief Justice of England and Wales, Baroness Carr of Walton-on-the-Hill, who introduced him to members of the judiciary.

The King also viewed an exhibition on female judges and heard about the work of voluntary magistrates.

Addressing a reception of judges and magistrates in the building’s Painted Room, Charles said: “Maybe I’d be permitted just to thank you all enormously for the amount of effort you put into maintaining the system of justice in this country, which so many others seem to be envious of I discover when going around the world.

“I cannot thank you enough.”

His visit came a day before his youngest son Harry is due to find out the outcome of his unlawful information gathering claim against Mirror Group Newspapers (MGN) in a High Court ruling.

The Duke of Sussex sued the publisher for damages, claiming journalists at its titles – the Daily and Sunday Mirror and Sunday People – were linked to methods including phone hacking, so-called “blagging” or gaining information by deception, and use of private investigators for unlawful activities.

Mr Justice Fancourt, the judge who oversaw a trial of the claims earlier this year, is expected to give his ruling at a hearing on Friday.

Harry faced eight hours of questioning over two days during a witness box appearance.

MGN largely contested the claims and denied that any newspaper articles complained of resulted from phone hacking, while contending that the vast majority did not arise from any other unlawful activity.

Harry is currently involved in five cases at the High Court.