BRITNEY SPEARS is starving herself on an ex­treme 600-calories-a-day diet – and the side effects are causing a stink on the “X Factor” set.

“Britney’s breath absolutely reeks!” said a close source familiar with the odor.

“It must be the acids in her tummy or something from this cra­zy diet she’s on because her breath will start smelling 20 minutes after she brushes her teeth.”

The “potty-mouth” entertainer, who’s joined the talent competi­tion as a judge, is on a strict diet to look her best for the soon-to-air live shows.

But her bare-bones regimen is “absolutely brutal,” reveals a friend.

“Every day she eats four egg whites for breakfast, a few hours later she has a whey protein shake made with water and a couple of strawberries. Lunch is a half a steamed chicken breast and a salad, then another shake, and for dinner more lean meat with veg­gies. It’s barely 600 calories a day.”

Dr. Craig Title, a New York weight-loss specialist, explained that a diet very high in protein and low in fats and carbohydrates can cause bad breath by producing breath-affecting byproducts or ammonia in the body.

It’s not the first time Britney’s been accused of assaulting noses.

A lawsuit filed last year by her former bodyguard Fernando Flores alleged she had bad hygiene, and in 2004 fel­low airline passengers complained to stewards about her smelly feet.