Freddie Prinze Jr. just UNLOADED on Kiefer Sutherland -- saying he HATED working with Kiefer on "24" and calling him the "most unprofessional dude in the world."
Freddie made the comments at Comic-Con -- telling ABC News Radio his experience on "24" in 2010 was so bad, he wanted to quit acting altogether.
"I did '24' ... it was terrible. I hated every moment of it. I just wanted to quit the business after that. So, I just sort of stopped."
He then turned his sights to Sutherland -- saying, "Kiefer was the most unprofessional dude in the world. That's not me talking trash, I'd say it to his face. I think everyone that's worked with him has said that."
In fact, Prinze Jr. still hates Kiefer so badly, he took a shot at the actor for being short -- saying, "I went and worked for Vince McMahon at the WWE, for Christ's sake … but, at least he was cool and tall. I didn't have to take my shoes off to do scenes with him, which they made me do [at 24]. Just put the guy on an apple box or don't hire me next time. You know I'm 6 feet and he's 5'4."
He clearly changed his tune ... back in 2010 Prinze had nothing but good things to say about working with Kiefer.