Harry applied for Judicial Review when the decision was taken to withdraw protective security and not allow the Duke to pay for police protection for him and his family when they return to Britain.
The row follows his and wife Meghan’s decision to step back from their roles as working Royals and move to the US to live as private citizens.
At the High Court on Friday, Mr Justice Swift agreed Harry had an “arguable” case to challenge the February 2020 decision of the Executive Committee for the Protection of Royalty and Public Figures (Ravec).
He concluded the committee is entitled to make decisions independently on Royal security, but Harry can challenge an alleged “procedural unfairness” and the transparency of the decision-making process.
“Ravec has not had the opportunity to explain the reasons for its decision”, said the judge in his ruling. “The court will not be able fully to appraise the merits of this ground of challenge until (it’s) evidence is available.”
Harry has been granted permission to challenge the Home Office on four grounds, including that he was not able to make representations directly to Ravec before the decision was taken to reduce his security entitlement.
And he can bring a claim that he “should have been told the contents of Ravec’s policy before the February 2020 decision was made”, the judge said.
The judge also refused permission for Judicial Review on other parts of the Duke’s application, including that Ravec’s policy was applied in an “overly rigid and inflexible manner”.
“In approaching its own task of deciding which persons in what circumstances can be provided with protective security, Ravec was, subject to the usual public law standards, entitled to decide for itself which persons within the line of succession to the Crown should fall within” a certain category”, the judge concluded.
The case now moves forward with the Home Office putting forward its defence to Harry’s claim.
At the previous court hearing, the Duke’s barrister Shaeed Fatima QC argued Harry had been out of the loop when decisions on his security were made.
She said he did not know members of the Royal Household were involved, believing it was “an independent decision”.
Lawyers for the Home Office say Ravec was entitled to reach decisions on a “case by case” basis.
Parts of Friday’s ruling have been redacted by the judge, as they deal with specific matters relating to Harry’s security and were heard by the court in closed sessions.