During the first day of the prince's legal action against Mirror Group Newspapers (MGN) the High Court was told his ex-girlfriend Chelsy Davy decided "a royal life was not for her" as a result of alleged unlawful information gathering by journalists.

In court documents released at the start of the trial, MGN admitted there was "some evidence of the instruction of third parties to engage in other types of UIG (unlawful information gathering) in respect of each of the claimants", of which the Duke of Sussex is one.

The publisher denies involvement in the case of one claimant - Coronation Street actor Michael Turner (Michael Le Vell) - but the apology extends to Corrie actress Nikki Sanderson and the ex-wife of comedian Paul Whitehouse.

MGN also said the evidence "warrants compensation".

The publisher added: "MGN unreservedly apologises for all such instances of UIG, and assures the claimants that such conduct will never be repeated."

The statements form part of the publisher's defence to claims brought by Harry and the other individuals over alleged unlawful information gathering at MGN titles.

Representing the publisher, Andrew Green KC said in the written arguments that it denied allegations of voicemail interception in the cases being examined during the trial.

Mr Green also said some of the challenges it faces have been brought beyond a legal time limit.

The barrister said: "There is some evidence of the instruction of third parties to engage in other types of UIG in respect of each of the claimants" except for the case of actor Michael Turner whose claim is "entirely denied".

He added: "This apology is not made with the tactical objective of reducing damages, MGN accepts that an apology at this stage will not have that effect, but is made because such conduct should never have occurred."

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Mr Green said the publisher "unreservedly apologises" for one instance of UIG against the duke, adding MGN admitted to using a private investigator to unlawfully gather information about what he was doing at the Chinawhite club in 2004.

"Otherwise, the specified allegations are denied, or in a few cases not admitted," he added.

Mr Green also said there was a reference to a payment record for £75 in February 2004.

"MGN does not know what information this related to, although it clearly had some connection with his conduct at the nightclub."

Mr Green added: "The Duke of Sussex notably does not claim in relation to this article, so it is not alleged that this instruction led to the publication of his private information.

"The fee paid, £75, suggests little work was involved."

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Elsewhere in court documents, David Sherborne, representing the Duke of Sussex and the other individuals bringing claims against MGN, alleged the use of unlawful information gathering used by the publisher was "habitual and widespread".

"The scale and extent of the methods were used so frequently they were the stock-in-trade of journalists."

Mr Sherborne also alleged in court that the use of private investigators was authorised by senior editors, including Piers Morgan, adding the former editor, and MGN's legal department, "must have known" that they were using "illegal methods and still continued to use them".

"Put bluntly it was worth the risk," Mr Sherborne told the court.

"It was a risk because it contradicted the very public denials that they were making in relation to the use of unlawful information gathering."

Mr Morgan has previously denied any wrongdoing.

Mr Sherborne also told the court, in a written submission, details about the alleged phone hacking of Prince Harry.

He said the duke "experienced unusual telephone and media-related activity which is consistent, now in hindsight but at the time unsuspected, with the unauthorised accessing of his voicemails and other unlawful information gathering".

Mr Sherborne added this included missed and hung-up calls "on an almost daily basis from numbers he did not recognise".

He said these activities caused the duke "huge distress" and created "a huge amount of paranoia in my relationships".

Referencing Prince Harry's witness statement in the case, Mr Sherborne added: "He became immediately suspicious of anyone named in stories about him and felt that he could not trust anybody, even at such a young age. It also caused great challenges in his relationship with his ex-girlfriend Chelsy Davy, and made him fear for his and her safety ...

"Ultimately, MGN's activities led Ms Davy to make the decision that 'a royal life was not for her', which was 'incredibly upsetting' for the Duke of Sussex at the time. They also caused their circle of friends to become smaller and smaller, meaning that friendships were lost entirely unnecessarily, and led to 'huge bouts of depression and paranoia'."

Prince Harry, as well as the other claimants, is expected to give evidence to the High Court in June.

It will be his second appearance in the building this year, after sitting in a preliminary hearing for a separate claim against Associated Newspapers.