EXCLUSIVE

Johnny Depp's disdain for the White House runs deep ... so deep he was once corralled by Secret Service agents who refused to let him shake the President's hand.

Depp, who's taking heat for his Trump assassination crack, was at 1600 Pennsylvania Ave. back in January 1988 -- yeah, we know it's the '80s -- where President and Nancy Reagan held a ceremony for the fight against AIDS.

Johnny -- who was the star of "21 Jump Street," started clowning almost from the get-go. During a private tour, he picked his nose at the podium in the Press Room and flipped off the camera outside the gate.

We're told he was in the Rose Garden when he lit up a cigarette, much to the dismay of the Secret Service.

Later, during the event, Johnny was in the Blue Room when he once again lit a ciggy ... enraging the agent.

When it came time for the receiving line to meet the Prez and First Lady, the agents told Johnny he could not participate because he had shown disrespect.