Looks like Billy Joel's upcoming biopic is headed for "The Longest Time" in development hell -- 'cause the icon himself is coming out swinging against it!

A rep for the "Piano Man" says the filmmakers behind "Billy and Me" have known since 2021 they were never getting the green light, and still don't have the rights to his music or his story to make the movie fly.

Billy ain't exactly keeping the "Pressure" low over the John Ottman-directed flick ... the statement to Variety says he "has not authorized or supported this project in any capacity" ... warning any attempt to move forward would be "legally and professionally misguided."

The flick focuses on Billy before the fame, before the hits ... and before "Piano Man" turned him into a superstar in 1973.

The story's told through the eyes of Billy's first manager, Irwin Mazur, who discovered him in the '60s and helped steer his rise before Columbia Records signed him.

Casting's apparently underway already ... but without Billy's blessing -- and most importantly, the rights to the music that made him famous -- this thing is hitting a serious sour note before cameras even roll.