There's a big, fat problem on the set of "Biggest Loser" -- TMZ has learned nearly all of the remaining contestants have walked off the show and are threatening to quit after learning about a secret "twist" that could seriously affect their shot at the $250,000 grand prize.
Multiple sources connected with the show tell us ... the mutiny took place this week ... when the losers got word that producers were considering bringing back previous contestants to compete for the grand prize.
We're told most of the current contestants were pissed -- feeling it was unfair that prior losers would get a second bite at the apple ... so they banded together and decided to protest by walking off the show in the middle of shooting an episode.
Sources tell us ... show honchos have shut down production for at least a week while they figure out how to handle the situation.
We've reached out to "Loser" and NBC for comment -- so far, no response.
