During a sit down with GBN broadcaster Nigel Farage, the ex-Prez made it clear that King Charles' son would not get special privileges regarding his residency in the States if he potentially lied on his visa application.

Remember, Harry was very candid about his past drug use -- including cocaine, weed and psychedelic mushrooms -- in his memoir "Spare."

While PH's hard-partying ways seem far behind him ... an applicant's past drug use may technically make them ineligible for a visa -- which is why many are curious about what Harry dished on his paperwork.

In response to the chatter, Donald has promised to take action if the Duke's visa had any inaccuracies -- but stayed coy on the specifics of his plan ... just like he had before.

Remember, this ain't the first time DT has suggested Harry may get the boot under a new potential term of his -- but he isn't outright saying the guy's got something to worry about.

Prince Harry currently resides in the U.S. on a visa, as he's yet to become an American citizen -- something he says he is still considering. His wife, Meghan Markle, is a natural-born U.S. citizen with their children, Archie and Lilibet, being dual citizens.