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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.