A “unique” court case concerning the Duke of Sussex’s immigration records should be reopened because certain submissions by the US government were made in secret, it has been claimed.

A judge ruled in September that Prince Harry’s US visa application should remain private, following an almost two-year legal battle surrounding his admission of drug taking.

The Heritage Foundation, a Washington DC-based think tank, failed in its bid to access the documents on the grounds that the Duke waived his right to privacy when he divulged personal details in his memoir.

However, the foundation is now seeking to reopen the case on the basis that it was not granted access to private submissions made by the Biden administration to the judge.

In a new court filing, it states that the way the case was conducted breached “iron-clad guardrails” on conducting ex-parte proceedings – those involving only one party.

The foundation claims in a 13-page motion, lodged with the court on Tuesday, that certain documents should have been reviewed in court before declarations were sought and that ex-parte proceedings were held without being recorded on the court file.

“Collectively, these errors were not harmless,” the think tank argues.

The secrecy surrounding certain elements of the case left both the think tank and the public in the dark about the reasons behind the judge’s ruling, it claims.

The “most obvious consequence”, it argues, is that its ability to prepare arguments on appeal is “severely compromised”.

The motion acknowledges that the “unique” case was fraught with complexities but warns there is “ample evidence of agency bad faith”.

It calls on the court to “vacate its opinion and order, enter all ex-parte correspondence on the docket, unseal ex-parte correspondence consistent with the opinion” and to allow the further raising of any legal issues.

The Heritage Foundation brought a lawsuit against the Department for Homeland Security (DHS) after it rejected a Freedom of Information Act request for access to the Duke’s records.

It questioned how the royal had been able to relocate to the US after he admitted to taking cocaine and other illegal drugs in his book, Spare.

But Judge Carl J Nichols ruled that “the public does not have a strong interest in disclosure of the Duke’s immigration records”.

He added: “Like any foreign national, the Duke has a legitimate privacy interest in his immigration status.

“And the Duke’s public statements about his travel and drug use did not disclose, and therefore did not eliminate his interest in keeping private, specific information regarding his immigration status, applications, or other materials.”

The Heritage Foundation could still appeal the ruling.

Visa applicants are legally obliged to declare whether they have taken drugs. While it does not constitute an automatic ban, failure to do so can lead to deportation.

Anyone classed as a “drug abuser” is in danger of being deemed “inadmissible” and celebrities including Nigella Lawson have been prevented from entering the country after admitting drug use.