Thames Valley Police has accepted that the search warrants it used to search Andrew Mountbatten-Windsor's homes earlier this year were unlawful, the High Court has heard.
The force said it accepted that the "initial court's decision to issue warrants under the specific piece of legislation used was incorrect" before officers searched the former prince's properties on 19 February.
Andrew Mountbatten-Windsor, 66, the younger brother of King Charles, was arrested by Thames Valley Police on suspicion of misconduct in public office. He denies any wrongdoing.
The criminal investigation is ongoing. Items seized during raids on his homes in Norfolk and Berkshire will remain in police custody while his legal challenge over the material continues.
Andrew's lawyers have used a judicial review to challenge the actions of the force's chief constable and the Central Criminal Court.
His arrest in February followed the publication of documents by the US Department of Justice which suggested he had forwarded confidential information to convicted sex offender Jeffrey Epstein while serving as a UK trade envoy in 2010. He has consistently denied any wrongdoing in relation to Epstein.
The court heard on Thursday that Thames Valley Police took possession of documents and electronic devices during the raids.
The force had applied for search warrants under section eight and section nine of the Police and Criminal Evidence Act 1984 at a hearing at the Central Criminal Court the day before the search.
That court determined that only section eight warrants were needed and granted them on that basis. A subsequent legal challenge established that the warrants should have been issued under section nine instead.
The conditions for the warrants required reasonable grounds for believing that documents used for business purposes, or in a paid or unpaid role in which a duty of confidence had been undertaken, should not be seized.
Thames Valley Police now admits the judge who granted the warrants could not have been satisfied that this condition was met.
