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Baby, baby? No.

Justin Bieber and his team are swinging back at a shocking new lawsuit claiming he fathered a child with a 20-year-old California woman.

According to Star magazine, Mariah Yeater says she and the teen pop superstar, 17, had sex backstage at an L.A. show; three months ago, she gave birth to a baby boy, and has filed a paternity lawsuit against Bieber, and asked for financial support from the singer.

In response, a rep for Bieber tells Us Weekly in a statement:

"While we haven't yet seen the lawsuit, it's sad that someone would fabricate, malicious, defamatory AND demonstrably false claims. We will vigorously pursue all available legal remedies to defend and protect Justin against these allegations."