Prince Harry is under pressure to reveal the answers he gave on his US visa application regarding his previous drug use.
Visa applicants are required to tick a box to answer ‘yes’ or ‘no’ to the question: ‘Are you or have you ever been a drug abuser or addict?’
A Freedom of Information (FoI) request has been filed by a leading US think tank who are asking the Duke to be "totally transparent" about his answers.
Harry has openly admitted to using a variety of substances in the past, including marijuana, cocaine and magic mushrooms.
In his best-selling memoir, Spare, the royal recalled using psychedelic drugs.
He reaffirmed the drug usage last month while taking part in a 'therapy session' with toxic trauma expert Dr Gabor Mate.
The drugs he took included the hallucinogenic Amazonian plant ayahuasca, which supposedly has the effect of "removing life’s filters".
Admission of drug use usually results in visa applications being denied.
Senior lawyer Samuel Dewey at conservative think-tank the Heritage Foundation, which has filed a 127-page FoI request to see the Duke’s visa application, told the Mail on Sunday: "It is in the public interest to know how Prince Harry answered the drug question.
"If he has been honest and open about his drug use, and there is no reason to believe he has not been, it could well be that he ticked the “yes” box, in which case he would need a waiver to be granted a visa to be admitted into the States.
"That means he would have had to be interviewed in person and someone would have had to grant him a waiver. We are simply asking who granted that waiver."
Mr Dewey highlighted that an initial visa rejection does not always mean a lifetime barring from the US.
A ban is sometimes overturned following an in-person interview at a US consulate or official immigration office, where a waiver can be issued.
It comes after America's Heritage Foundation called for Harry's visa application to be released so the US taxpayer can understand whether the royal declared his drug use.
