Andrew Mountbatten-Windsor has won his legal challenge against Thames Valley police over search warrants that led to his arrest.

The former prince brought a judicial review of the legality of the force’s actions and those of the central criminal court, which granted the warrants.

On Thursday, Mr Justice Hilliard, sitting at the high court in central London, said Thames Valley police had conceded that it obtained the search powers under the wrong statutory provision and ruled that the warrants were unlawful.

The warrants were issued by the recorder of London at the Old Bailey on 18 February and Mountbatten-Windsor was subsequently arrested the following day, on his 66th birthday, on suspicion of misconduct in public office.

The arrest followed information that emerged from the Jeffrey Epstein files. It was alleged that when Mountbatten-Windsor was the UK’s trade envoy he shared sensitive information with Epstein, a convicted child sex offender.

Police began searching the former Duke of York’s residence on the Sandringham estate and his former Windsor home, Royal Lodge, on the same day as his arrest. Documentary material and electronic devices were seized.

Hilliard said the search warrants were quashed but police had made an application, which would be heard next year, to retain the seized property on the basis that it would have been lawful to take it under a different statutory provision. Mountbatten-Windsor opposes that application.

Police officers patrolling near to Royal Lodge, Andrew Mountbatten-Windsor’s former residence in Windsor Great Park, on 19 February. Photograph: Leon Neal/Getty

The judge said: “The fact that the search warrants were quashed does not bring the investigation to an end or mean that the whole of the investigation is unlawful.”

A Thames Valley police spokesperson said: “The agreement that led to the quashing of the warrants relates specifically to a decision made by the court regarding the legal basis for these and is not a judgment on our ongoing investigation.

“We ensured the court was presented with all the necessary information and paperwork to inform their decision, following the correct legal processes throughout.

“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.”

They added that the investigation into alleged misconduct in public office “remains ongoing, and we continue to pursue all reasonable lines of inquiry to effectively and fairly progress this investigation”.

Hilliard told the court that the grant of a search warrant under section eight of the Police and Criminal Evidence Act (Pace) 1984 was conditional on the issuing judge being satisfied there were reasonable grounds to suspect that the material sought did not include items subject to legal privilege or special procedure material.

Special procedure material includes documents in possession of a person who acquired them in the course of any trade, business, profession or other occupation – for the purpose of any paid or unpaid office – who holds it subject to an express or implied undertaking to hold it in confidence.

The judge said the presence of legally privileged material was expressly envisaged in the application for the search warrants and that material was likely to be in the possession of Mountbatten-Windsor “who had acquired or created it in circumstances which would make it special procedure material”. He concluded that a lawful warrant could only have been issued under section nine of Pace.

Hilliard also said the force had agreed that documents relating to the judicial review, which it had previously wanted to keep secret, could be released but with the names of officers and other staff working on the investigation removed as well as that of a legal representative.

Late last year, King Charles stripped his brother of his right to be a prince and of his dukedom over his association with Epstein.

Mountbatten-Windsor receives an annual stipend, the amount of which has not been disclosed, from King Charles’s private funds, but a palace source told reporters it had been made clear this should not be used to fund the judicial review.