Donald Trump has branded prince Harry "unforgivable" after "betraying" the late Queen as the former president slams the Joe Biden administration for "protecting" the Duke.

Speaking at the CPAC conference in Washington DC, Trump suggested that the Royal Family has been "too gracious" with Harry.

The 45th US President added that he would not be "protecting" him like Biden amid a dispute over his immigration application.

Prince Harry appeared in court on Friday after a conservative think tank took legal action to demand Harry’s US immigration files are made public.

In his memoir Spare, the prince, 39, admitted to formerly taking marijuana, cocaine and psychedelic mushrooms.

Drug use can be used as grounds to deny a US visa application, which could threaten his US immigration status.

The Heritage Foundation sued the Department of Homeland Security under the Freedom of Information Act claiming that Prince Harry may have received favourable treatment when applying for his visa.

The legal challenge has been resisted by the prince with the support of the Biden administration.

"I wouldn’t protect him. He betrayed the Queen. That’s unforgivable. He would be on his own if it was down to me," Trump told the Daily Express US.

"I think they have been too gracious to him after what he has done."