Justin Bieber -- I Didn't Knock Up That Woman

Justin Bieber's legal team is ready to strike over allegations the singer impregnated a 20-year-old woman ... allegations that Team Bieber claims are "malicious, defamatory, and demonstrably false."

The woman in question reportedly filed a paternity suit against Justin, claiming she boned the underage singer backstage at one of his L.A. concerts ... and left with an embryonic party favor.

But a rep for Justin tells TMZ, "While we haven't yet seen the lawsuit, it's sad that someone would fabricate malicious, defamatory, and demonstrably false claims."

The rep adds, "We will vigorously pursue all available legal remedies to defend and protect Justin against these allegations."