Justin’s reps deny the woman’s accusation that she slept with him in a bathroom backstage after one of his concerts in 2010.

A 20-year-old woman has filed a paternity lawsuit against Justin Bieber in San Diego, Calif., claiming he is the father of her three-month old son. Star is reporting that Mariah Yeater says she slept with Justin on Oct. 25, 2010, when she was 19 — and he was only 16. “[Justin] told me he wanted to make love to me and this was going to be his first time,” she claims in the lawsuit.

The conception allegedly went down backstage after one of Justin’s concerts at the Staples Center in Los Angeles. Mariah’s suit claims Justin refused to use a condom.

“Justin suggested that I go with him to a private place where we could be alone. … After walking away from the other people backstage, Justin found a place where we could be alone — a bathroom.”

A Justin Bieber team insider told Hollywoodlife.com that “the lawyer has been peddling this story for money to a number of outlets. It really raises serious questions about the lawyer’s motive. Justin is naturally a target. And of course, the accusations are total B.S.!”

Justin’s PR rep, Matt Hiltzik, also issued this statement:

“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.”

We’ll have more on this story as it unfolds, but it sounds totally crazy to us! Poor Justin!

UPDATE 11/4: Justin Bieber and Selena Gomez Break Up After Baby Scandal, Says Report

UPDATE 11/2: Mariah Yeater PHOTOS here

Click Here To See The Baby’s Birth Certificate on E! Online!