Jake 'Raging Bull' LaMotta -- Sued Over Sequel Flick ... It's a Cheap Knockoff!!!

Boxing legend Jake "Raging Bull" LaMotta -- played by Robert DeNiro in the famous movie of the same name -- is being sued by MGM Studios because Jake green-lit a crappy sequel without its permission.

MGM claims it snagged the rights to produce a movie based on a book LaMotta co-wrote in 1986 -- "Raging Bull 2" -- as a sequel to the 1980 hit movie. MGM claims the deal was ... Jake could take the project somewhere else ONLY if MGM passed.

But according to the lawsuit filed in L.A. County Superior Court -- LaMotta went behind MGM's back and shot a "low-budget B movie" that is so bad, it threatens the "integrity and value" of the original.