Funnyfellas WILL FERRELL and ZACH GALIFIANAKIS started their new flick “The Campaign” as good friends – but now literally “LOATHE each other!”

Reports my backstage spy, Onda Settalot: “It started off fine, but things quickly went downhill when shooting started. It’s just a classic clash of egos!

"Will thinks he’s the bigger star, and resents that Zach keeps bragging that he’s being paid nearly $15 million for his role – far too much, in Will’s opinion.

"Very quickly, they stopped talking to each other and studio publicists were instructed to schedule them together for PR events only when absolutely necessary. They totally despise each other!”

Even at the Hollywood Grauman’s Chinese Theatre premiere after-party, Will and Zach pointedly stayed on opposite sides of the room, surrounded by handlers.

NOTE TO DUELING DUO: Critics attribute your flick’s lame box office to a lousy script, but…any chance audiences simply sensed your cranky, crappy ’tudes and were turned off? Remember who’s really paying your paychecks, big shots!