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UFC champ Amanda Nunes says it would be "impossible" for Claressa Shields to beat her in an MMA fight ... and she's warning the boxing star to back down before she gets hurt.

Shields is the biggest star in women's boxing by far -- she's undefeated, holds multiple belts and Olympic gold medals. But, she REALLY wants to fight Amanda and recently started MMA training in hopes of making it happen.

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The good news for Shields ... Dana White told us he's interested.

The bad news for Shields ... Amanda says she would SQUASH Claressa like a bug.

In fact, Amanda's fiancee Nina Ansaroff -- the #5 ranked UFC women's strawweight fighter -- thinks Shields is in for a rude awakening about the difference between MMA and boxing.

"She would have to win enough fights to even get into the UFC to even see Amanda. I don’t think that’s possible. Because there’s girls now that are coming from a wrestling background."

Amanda added, "We can get a girl from low rank right now in 135 and put her to fight. [Shields] wouldn’t pass the 1st round."

More bad news for Shields ... if an MMA fight doesn't happen, Nunes says she has ZERO interest in taking a boxing match with Claressa.

"I'm not a boxer, I’m an MMA fighter. If I wanted to be a boxer, I would be."