Police raids on Andrew Mountbatten-Windsor’s royal homes before his arrest were unlawful, the High Court has heard.

The former prince, who was not in attendance at the hearing, launched a legal claim against Thames Valley Police over the lawfulness of search warrants granted to them by a judge at the Old Bailey In February.

The warrants allowed officers to raid his residence on the Sandringham Estate and his former Windsor home, Royal Lodge, ahead of his arrest on suspicion of misconduct in public office, following claims he shared sensitive information with late sex offender Jeffrey Epstein during his time as the UK’s trade envoy.

Mr Justice Hilliard said a “significant error” was made in the application for the warrants and that was “enough to render the warrants, the searches and the material seized unlawful.

The judge said that while the former Duke of York and Thames Valley Police had reached an “agreement” that they were unlawful, police have since launched a bid to retain evidence gathered during the searches.

Mr Mountbatten-Windsor opposes that bid and a second hearing in the case will be held in private in 2027 to decide if the force can keep the seized material. It means that the former prince could remain in limbo for several more months before learning if he will be charged with an offence.

Thames Valley Police said in a statement issued after the ruling that their criminal investigation into “alleged misconduct in public office by a man in his sixties from Norfolk remains ongoing”.

That was confirmed by the judge, who said: “The fact the search warrants have been quashed does not bring the investigation to the end or mean the whole investigation is unlawful.”

The High Court had quashed the search warrants in July, which a police spokesperson explained “relates specifically to a decision made by the court regarding the legal basis for these and is not a judgement 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,” they added.

“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.

“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.”

The former Duke was arrested on suspicion of misconduct in a public office on his 66th birthday over allegations he shared sensitive information with Epstein while working as the UK’s trade envoy. He was interviewed under caution before being released. Mr Mountbatten-Windsor has always strenuously denied any wrongdoing.

The King’s younger brother then challenged the search warrants at the High Court, prompting Thursday’s hearing.

The court heard the warrant granted to police back in February did not cover material that is gained or created in the course of any trade, business, profession, or any paid or unpaid office, and which is expected to be confidential. This could include material Mr Mountbatten-Windsor obtained during his government-appointed role.

Mr Justice Hilliard said the presence of such material was “inherent” in the nature of the misconduct in a public office offence and therefore the application was made in error.

Danielle Reece-Greenhalgh, partner at Corker Binning, said that an unlawful or defective search warrant does not stop the police from examining the items during their investigation, and does not automatically mean any evidence cannot be used in any future trial.

She continued: “The question of whether and how unlawfully obtained evidence can be used centres on whether its admission would have such an adverse effect on the fairness of the proceedings that the court ought not to admit it.

“If the irregularity or unlawfulness goes hand in hand with some significant resulting unfairness which is incapable of remedy, Mr Mountbatten-Windsor may have grounds to argue that whatever material has been uncovered should not be used against him.”

Misconduct in public office is a criminal offence that concerns serious wilful abuse, or neglect, of the power or responsibilities pertaining to the public office held.

Thames Valley Police previously said officers were continuing to assess a wide range of aspects covered under the criteria as part of its investigation.

Mr Mountbatten-Windsor has rarely been seen in public after he was forced to leave Royal Lodge in February and move to the Sandringham Estate in Norfolk.

The King stripped his brother of both his right to be a prince and his dukedom over his association with Epstein in October last year.