A British tabloid newspaper has apologised to Prince Harry for unlawfully seeking information about him at the start of the royal’s lawsuit against its publisher over alleged phone-hacking, in which he is due to give evidence himself.

Harry, 38, and some 100 celebrities, including actors, sports stars, singers and TV personalities, are suing publisher Mirror Group Newspapers (MGN), accusing its titles of habitually accessing private information by widespread phone-hacking, deception and other illicit means between 1991 and 2011.

The claimants say the unlawful behaviour at the Daily Mirror, Sunday Mirror and Sunday People had occurred with the full knowledge of senior executives, who they say failed to stop it and actively covered it up.

The titles are owned by newspaper company Reach PLC.

MGN is contesting the allegations, saying some claims have been brought too late, and that there was no evidence Harry was a victim of hacking. It denies any senior figures had knowledge of unlawful acts.

However, in documents to the High Court in London, MGN admitted on one occasion a private investigator had been engaged to unlawfully gather evidence about him at a nightclub in 2004, saying it “unreservedly apologises and accepts that (Harry) is entitled to appropriate compensation”.

Harry, who was not present for the start of the hearing, has been selected as one of four test cases for the seven-week trial and is due to give evidence himself in person in early June, the first British royal to do so since the 19th century according to local media.

“Prince or not, the blatantly unlawful and illegal methods that were used by the defendant … was quite frankly appalling,” David Sherborne, the lawyer representing Harry and the other claimants said.

Mr Sherbourne said the evidence would shoe “a constant theme of concern” from senior MGN management over the legality of their information gathering, “yet time and time again they are still commissioning”.

He pointed to email chains discussing the methods of information gathering, referring to “illicit” checks and “dodgy stuff”.

One email exchange from 2002 shows editorial manager of MGN, John Honeywell, flagging the “frightening” amount of money being spent on hiring private investigators.

Other senior management staff implicated by the claimants’ case include Piers Morgan.

Mr Morgan has always strenuously denied the allegations.

MGN’s defence to the claims will be heard at a later date.

The trial began just days after the coronation of King Charles III on Saturday where Harry appeared only briefly for the ceremony at London’s Westminster Abbey and played no formal role.