Andrew Mountbatten-Windsor and Thames Valley Police have agreed that search warrants that allowed officers to raid two of his properties earlier this year were unlawful, a High Court judge has said.

But the police force is seeking to keep material seized in the unlawful raids.

The former Duke of York is taking legal action against the force over the lawfulness of search warrants issued by a judge at the Old Bailey allowing officers to raid his residence on the Sandringham Estate and his former Windsor home, Royal Lodge, in February.

Warrants for the searches are understood to have been granted under the wrong section of the Police and Criminal Evidence Act 1984 at a hearing at the Central Criminal Court in February.

On Thursday at the High Court, Judge Mr Justice Hilliard said Mountbatten-Windsor and the police had agreed that the search warrants had been unlawful.

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

He said Andrew opposes the bid to retain the evidence, with a hearing in the case to be held in private in 2027.

It means that the former prince could remain in limbo for several more months before learning if he will be charged with an offence.

A Thames Valley Police spokesperson said: “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.

“Our investigation into alleged misconduct in public office by a man in his 60s from Norfolk remains ongoing, and we continue to pursue all reasonable lines of enquiry to effectively and fairly progress this investigation.”

Andrew was arrested on suspicion of misconduct in public office following allegations he shared sensitive information with Jeffrey Epstein during his time as the UK’s trade envoy.

He was interviewed under caution before being released under investigation, and denies wrongdoing.

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

But he also said: “There would be little point handing it back if it could be lawfully seized on some other provision.

“I will have to decide whether the new application is granted or not.

“The fact the search warrants have been quashed does not bring the investigation to the end or mean the whole investigation is unlawful.”

Andrew did not attend the hearing, with his barrister making no submissions.

The judge said police seized “documentary material and electronic devices” during the searches, and that the force “can still retain the property if it would be lawful to seize the property under some other provision once it was returned to the owner”.

The legal definition of misconduct in a public office can include a wide range of conduct – including sharing confidential financial information, financial misconduct, wilful neglect of duty, and sexual misconduct.

It also covers corruption and improper interference, conflict of interest, perverting the course of justice, dishonesty or fraudulent conduct, and misconduct leading to personal gain.

Police began their searches of the two properties and arrested Andrew on February 19, his 66th birthday.

Thames Valley confirmed on February 24 that their searches had ended.

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

A Buckingham Palace source confirmed on Monday that Andrew had been told not to use money from the annual stipend provided from the King’s private funds to finance the legal action.

Sean Caulfield, a partner at law firm Hodge Jones and Allen, said the lawfulness of a warrant can be challenged for several reasons.

He said: “The police may have gone beyond the remit of the search warrant; for example, there could be outbuildings or vehicles or secondary addresses that were not covered in the first grant of the search warrant.

“The judge could have misconstrued what he was told and gone beyond the test in the legislation in granting the search warrant, and it would therefore be invalid due to a mistake the judge made.

“There could also be some procedural errors in terms of dates and signatures on the form that would make it invalid.

“All of those in theory would result in a search warrant being in some way contaminated, but I do think all of those are quite unlikely.”

Mr Caulfield said he believed Andrew could receive compensation if successful, and the legal bill would already have run up to tens of thousands of pounds.