Speaking at the Conservative Political Action Conference near Washington DC on Saturday, Mr Trump told the Express: “I wouldn’t protect him. He betrayed the Queen. That’s unforgivable. He would be on his own if it was down to me.”

Prince Harry’s immigration status has become the subject of legal dispute by the Heritage Foundation, a conservative think tank, which argues he cannot legally have entered the US because he admitted to taking illegal drugs in his memoir, Spare.

Taking illegal drugs has previously precluded some foreign citizens, including Nigella Lawson, from travelling to the US.

The think tank argued in court on Friday that the US government should reveal Prince Harry’s immigration form, showing whether he declared he had taken the drugs to the Department of Homeland Security (DHS).

In his memoir, the Duke wrote that he had taken cocaine, marijuana and psychedelic mushrooms in his youth.

He said that while cocaine “didn’t do anything for me”, marijuana “really did help me”.

After taking psychedelic mushrooms, he said he had believed a bathroom bin had grown a head and was staring at him.

A lawyer for the Biden administration, disputing the challenge, told the court in Washington that the Prince’s admission of taking the drugs in his book was not “proof it had taken place.

The administration said his account could have been embellished to “sell books,” adding “saying something in a book doesn’t necessarily make it true”.

The Heritage Foundation has accused the US government of covering up the records, which it says are of “immense public interest”, to protect Prince Harry and his wife Meghan Markle, who is a US citizen.

Mr Trump has previously expressed strong support for the late Queen and has been critical of the Duke and Duchess.

He has described Prince Harry’s memoir as “horrible” and said he was “surprised” that the pair were invited to King Charles’s Coronation last year.

In response to the late Queen’s death in 2022, Mr Trump said: “What a grand and beautiful lady she was. There was nobody like her!”

Speaking in a television interview earlier this month, Prince Harry said he had not yet applied for US citizenship.

“The thought has crossed my mind but is not a high priority right now,” he said.