Freddie Prinze, Jr. -- Kiefer Sutherland Beef is Alcohol-Related ... Fox Says Hogwash

Freddie Prinze, Jr. believes alcohol made Kiefer Sutherland impossible to deal with on the set of "24" ... but a key Fox source tells TMZ Freddie's claims are BS.

Sources connected with Freddie tell TMZ ... the actor claims Kiefer would regularly show up on set drunk ... sitting in his trailer often for hours, as everyone waited. The sources say it messed with the lives of the family of cast and crew.

Freddie, we're told, claims Kiefer was temperamental and got people fired he didn't like, yet "24" producers consistently cow-towed to him ... as one source put it, "All they did was keep rewarding him."

Our sources claim Freddie was groomed to take over the series but it became such a nightmare for him he bailed.

Keifer's rep tells TMZ, "Kiefer worked with Freddie Prinze, Jr. more than 5 years ago, and this is the first he has heard of Freddie's grievances." The rep adds, "Kiefer enjoyed working with Freddie and wishes him the best."

A Fox official tells TMZ ... Kiefer was "nothing but professional during the run of the show and is beloved by cast and crew." The honcho adds, "It's so out of left field, 5 years later."

The official goes on, "We wouldn't have done another '24' if Kiefer were anything like Freddie described."