The US Government is set to be challenged in court over its decision not to disclose the reasoning behind admitting the Duke of Sussex into the country, despite his admissions of illegal drug use.

Harry’s reference to taking cocaine, marijuana and psychedelic mushrooms in his memoir Spare prompted a conservative Washington DC think tank to question why he was allowed into the US in 2020.

The Heritage Foundation brought the lawsuit against the Department for Homeland Security (DHS) after a Freedom of Information Act request was rejected – claiming it was of “immense public interest”.

The think tank is to argue the “widespread and continuous media coverage” has brought into question whether the duke was properly vetted by the US Government.

But the DHS said swift release of the documents would not be in the public interest and questioned how “widespread” the media coverage of the issue had been.

Both parties are set to argue the case in a federal court for the first time in Washington DC on Tuesday.

It is unclear when the court will make a decision over whether the documents can be released.

In his controversial memoir, the duke said cocaine “didn’t do anything for me”, adding: “Marijuana is different, that actually really did help me.”

The Heritage Foundation’s lawsuit argues that US law “generally renders such a person inadmissible for entry” to the country.

It comes as Harry is set to give evidence in the High Court in the UK in his individual case against Mirror Group Newspapers over alleged unlawful information gathering.