'Hell's Kitchen' Winner Ja'Nel Witt -- FAILED DRUG TEST

"Hell's Kitchen" champ Ja'Nel Witt tested positive for cocaine ... and that's why she wasn't allowed to accept the head-chef position at Gordon Ramsay's restaurant in Vegas ... TMZ has learned.

Talk about blowing a huge opportunity ...

Sources extremely familiar with the situation tell us ... Caesars Palace required Witt to submit to a drug test in order to take the job as head chef at the Gordon Ramsay Pub & Grill, the position she earned after winning Season 11 of "Hell's Kitchen" earlier this year.

FYI -- Caesars has a notoriously strict drug policy ... and requires all employees to submit to drug testing.

But Witt bombed her test -- one source tells us she "failed miserably" -- and when Caesars found out ... they quickly kicked her ass to the curb.

But there is a silver lining for Witt -- we're told she WILL receive the $250k salary ... despite the fact she won't spend a single day in the kitchen.

Fun Fact -- $250,000 can buy roughly 10 kilos of cocaine ... depending on the quality.