Letters between Diana, Princess of Wales and former television entertainer Michael Barrymore about their “highly sensitive meetings” show that her phone was hacked by the Daily Mirror, the High Court was told.

The trial over the Duke of Sussex’s allegations of unlawful information gathering against the newspaper’s publisher was told on Monday that his mother was a “huge target” for Mirror Group Newspaper’s (MGN) titles.

David Sherborne, Harry’s barrister, told the court that certain alleged unlawful activities in relation to Diana would have also affected the duke.

“It is part of our case that the interception of her messages would necessarily have involved obtaining information about the young prince,” Mr Sherborne said.

The lawyer referred to letters exchanged between Diana and former television personality Mr Barrymore.

Mr Sherborne read out two letters from Diana to Mr Barrymore, which referred to meetings between the pair, and in one of them Diana referred to being “devastated” to learn the “Daily Mirror” had called her office about him and their meetings.

In the letter, Diana said she had not told anyone about the meetings, the court heard.

Mr Sherborne said: “We say it is plainly that the Daily Mirror has been listening to the voicemail messages and that is how they knew of the secret and highly sensitive meetings between Princess Diana and poor Mr Barrymore.”

The barrister added that Mr Barrymore was “struggling” at the time because it had been made public that he was gay, he was in the process of an “acrimonious” divorce and was being treated for alcohol and drug addiction.

In one letter, Diana wrote to Mr Barrymore: “What a joy it was to finally meet you tonight and it was a privilege to share so much with you.

“I did want to emphasise that I’m here for you whenever, it’s very easy to pop round and see you or please telephone now you have my number any time.

“You’re doing just fine and believe me, I know. So take great care and lots of love from Diana.”

Mr Sherborne said former Mirror editor Piers Morgan referred to hearing rumours about the meetings between Diana and Mr Barrymore in his book The Insider.

The barrister said the reason Mr Morgan had heard was because Mirror journalists would have heard “private messages”.

He also said a letter to Mr Barrymore from Diana in June 1997, just two months before her death, “demonstrates the impact of these activities”.

Mr Sherborne said Diana wrote to Mr Barrymore saying she had not heard from him and hoped his silence was “good news”, adding: “I have had a nightmare time with the tabloids.”

The barrister added: “Mr Barrymore is so frightened off that he does not contact Princess Diana and this is the isolation that this activity causes.”

Andrew Green KC, representing MGN, later told the court: “Mr Sherborne’s suggestion that MGN’s journalists hacked her phone is total speculation without any evidential basis whatsoever.”

“The letters you were shown, to Michael Barrymore, are not evidence of voicemail interception,” Mr Green told the judge, adding that “plainly no such finding could be made”.

Mr Green also said there was “simply no evidence capable of supporting the finding that the Duke of Sussex was hacked” and that private investigator payment records in the case “simply do not demonstrate unlawful conduct or knowledge thereof”.