The Duke of Sussex’s claim over allegations of unlawful information gathering against the publisher of The Sun will go ahead to a trial, following a High Court ruling.

Harry, 38, alleges he was targeted by journalists and private investigators working for News Group Newspapers (NGN), publisher of The Sun and the now-defunct News Of The World, and has launched a claim for damages.

At a hearing in April, NGN asked Mr Justice Fancourt to throw out the duke’s case, arguing it was brought too late because he should have known sooner he had a potential claim.

In a ruling on Thursday, the judge concluded that Harry cannot bring his claim relating to phone hacking, but that his claim over other allegations – including use of private investigators – should go ahead to a trial which is due to take place in January next year.

The judge also refused to allow the duke to rely on an alleged “secret agreement” between the royal family and senior executives working for media mogul Rupert Murdoch as part of his claim.

A spokesperson for NGN said after the ruling that the judge had “dismissed” the duke’s phone hacking claims against its two titles in a “significant victory” for the publisher, adding that the judgment “substantially reduces the scope of his legal claim”.

In his written ruling, the judge concluded: “I am satisfied that there is no reasonable prospect of the duke proving at trial that he did not know and could not with reasonable diligence have discovered facts that would show that he had a worthwhile claim for voicemail interception in relation to each of the News Of The World and The Sun.

“He already knew that in relation to the News Of The World, and he could easily have found out by making basic inquiries that he was likely to have a similar claim in relation to articles published by The Sun.”

The judge also said that “there is no evidence currently before me” that the duke knew at least six years before he issued his claim “that NGN had done anything other than hack his mobile phone, at the News Of The World”.

“Knowing or being on notice of a worthwhile claim for voicemail interception does not of itself amount to knowledge or notice of a worthwhile claim for other forms of (unlawful information gathering),” he added.

The judge said a trial should determine whether Harry’s claim over other unlawful information gathering was brought within the six-year time limit for such claims.

Harry’s lawyers previously argued that NGN’s challenge to his claim was an attempt to go behind the alleged “secret agreement”, between the royal family as an institution and the publisher, which the duke was told of in 2012.

They said that, while he was aware of unlawful activity in around 2012, the duke had no reason to think it had taken place at The Sun, and was prevented from bringing a claim because of the alleged agreement.

But NGN, which denies any unlawful activity took place at The Sun, disputed that such an agreement was in place.

Mr Justice Fancourt said in his ruling that the duke had not “provided any evidence from those in the palace who would have been aware of a secret agreement if there was one”.

He said he was unable to conclude that there was “a sufficiently plausible evidential basis” for a case based on the alleged agreement to justify the duke amending his claim against NGN.

The judge said the “lack of credibility” over the alleged agreement came from “the improbability of a secret agreement being made” in the terms claimed; the “inconsistency” in the duke’s case and “the absence of any other witness or documentary evidence to support it”.

At a hearing in London earlier this month, lawyers for the duke said there was evidence to support the existence of the agreement, including emails between senior executives at the Rupert Murdoch-owned parent companies of NGN and palace staff in 2017 and 2018.

David Sherborne, for Harry, also said in written arguments the fact the Prince of Wales settled a claim against NGN “for a very large sum of money” in 2020 also “supports the contention that there was a secret agreement in place”.

But Anthony Hudson KC, for NGN, told the court the bid to alter the duke’s claim is a “radical intended revision” of his case and he is “trying to ride two horses galloping in completely different directions” and “hedge his bets”.

Mr Hudson said the alleged agreement was “such a secret agreement that no-one apart from the claimant knows anything about it, and even the claimant knows very little about it”.

The barrister also said there was an “extraordinary delay” between Harry launching his claim against NGN in 2019 and first raising the issue of the “secret agreement” following the publisher’s strike-out bid.

The NGN spokesperson said on Thursday: “As we reach the tail end of litigation, NGN is drawing a line under disputed matters, some of which date back more than 20 years ago.”

They said the judge had found Harry’s claims over the alleged agreement were “not plausible or credible”, adding: “It is quite clear there was never any such agreement and it is only the Duke who has ever asserted there was.”

Mr Justice Fancourt ruled in May that a claim by actor Hugh Grant over alleged unlawful information gathering – other than allegations of phone hacking – can go ahead to be tried next January.

Grant, 62, is suing NGN in relation to The Sun only, having previously settled a claim with the publisher in 2012 relating to the News Of The World.

NGN has previously settled a number of claims since the phone-hacking scandal broke in relation to the News Of The World, which closed in 2011, but has consistently denied unlawful information gathering took place at The Sun.

Harry has been involved in six legal battles at the High Court in recent months.

His civil litigation has seen him bring claims against three major newspaper publishers over allegations of unlawful information gathering, as well as legal challenges against the Home Office in relation to his personal security.