One error in a warrant application leaves police fighting to keep the Mountbatten-Windsor evidence

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."

Justice Nicholas Hilliard said significant errors were made by Thames Valley Police in applying for search warrants and by a judge in granting them.

"the fact that the search warrants were quashed does not bring an investigation to an end, or mean that the whole of the investigation was unlawful."

Mountbatten-Windsor was arrested Feb. 19 on suspicion of misconduct in public office and police searched two of his homes.

"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."

The investigation follows claims Mountbatten-Windsor shared sensitive information with Jeffrey Epstein during his time as the UK's trade envoy.

Police questioned Mountbatten-Windsor for 11 hours before releasing him under investigation, meaning he was neither charged nor exonerated.

Attorney Mark Stephens, who was not involved in the case, said the development "opens a door to Andrew Mountbatten-Windsor's lawyers to start to chisel away at some of the fruits of that unlawful warrant."

The warrants were executed on Mountbatten-Windsor's 66th birthday in February, and he was arrested the same day.

"For example, in America, if you have the fruits of an unlawful warrant, then all that evidence is out. But that's not the case in the U.K., so we've got a few more rounds to go here."

"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."

Hilliard said he still needs to rule on whether to return Mountbatten-Windsor's property because there is a provision by which police can retain unlawfully seized property for evidence.

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

The judge said the warrants were unlawful because the judge who issued them had not considered whether items being seized were subject to legal privilege or another protection known as "special procedure material."

The court heard the February warrant did not cover material gained or created in the course of any trade, business, profession, or any paid or unpaid office, and expected to be confidential; Mr Justice Hilliard said the presence of such material was "inherent" in the nature of the misconduct in public office offence and therefore the application was made in error.

"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."

Criminal lawyer Nick Vamos said it was "embarrassing" police had made an error in such a high-profile case: "This should have gone under so many noses before it went to court that it's really amazing that they still managed to make a basic error," he told the BBC.

Mountbatten-Windsor receives a stipend from the king's private funds, but Buckingham Palace has made clear those funds may not be used to fund his effort to challenge the search warrants.

The High Court formalised the quashing about five months after the searches.

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