British police have made the embarrassing concession that search warrants they secured to search ‌Andrew Mountbatten-Windsor’s properties were “unlawful”.

Thames Valley Police said on Thursday (UK time) that the warrants were not properly granted by the judge —  citing a court procedural error in which the wrong section of the legislation was used.

However, the investigation would continue into the former prince’s alleged leaking of government documents to Jeffrey Epstein and the material police had gathered from Mountbatten-Windsor’s properties would still be kept.

Mountbatten-Windsor was arrested on February 19 on suspicion of misconduct in public office. Police searched two of his homes in Sandringham, eastern England, and Windsor.

Mountbatten-Windsor was suspected of leaking confidential government documents to Epstein from his time as a British trade representative, ‌between 2001 ⁠and 2011.

Police have not charged him and Mountbatten-Windsor has denied any wrongdoing in relation to Epstein. He has also said he regrets their friendship.

On Thursday, High Court Justice Nicholas Hilliard said the police had made a significant error in applying for the warrant when they searched Mountbatten-Windsor’s properties.

His lawyers asked the High Court judge to invalidate the warrants under which those searches were conducted.

Hilliard said the parties had agreed to invalidate the warrants before the hearing on Thursday.

Mountbatten-Windsor was not in court for the hearing.

Thames ​Valley Police ‌said it accepted the warrants had not been properly granted.

“We accept ​the position that the initial court’s decision to issue warrants under the specific piece of legislation used was incorrect and therefore these ​must ​be legally quashed,” a spokesperson said.

“Legal proceedings are ‌currently ongoing to decide how material seized during the searches will be managed and whether we can lawfully retain it in support of our ongoing investigation.”

Lawyer Mark Stephens, of the Howard Kennedy firm in London, described the hearing as a preliminary skirmish in the broader legal battle.

“It’s the first step of prince Andrew and his lawyers saying, ‘Actually, this is tainted evidence. It was obtained inappropriately’,” he said.

“Therefore it should be excluded from any criminal trial.”

The King stripped Mountbatten-Windsor of his princely title last year as he sought to shield the rest of the royal family from the continuing fallout from his younger brother’s links to Epstein.

-with AAP/AP