Justin Bieber is moving full speed ahead with a paternity test in a showdown with Mariah Yeater, the California woman who claims the teen sensation fathered her baby in a backstage tryst last year. Yeater insists she has the evidence.

Obviously both can’t be right, yet so far neither is blinking.

Bieber, 17, ramped up the pressure today after his rep, Matthew Hiltzik, confirmed for MTV News that a test has been scheduled without the need of a court order, and his team is vowing to sue Yeater and others, presumably her lawyers.

Check out Mariah’s photos; click to enlarge.

“I can confirm that the team has proactively made arrangements for Justin to be tested upon his return and we’re going to hold everyone who is involved responsible,” Hiltzik said.

The move comes after Yeater’s legal team announced that a top lawyer who once advised the White House on paternity law has joined them to assist in Yeater’s case.

Her new attorney, Jeffery Leving, is a noted Fathers’ Rights attorney and he is the President of the Illinois Council on Responsible Fatherhood, according to gossip site radaronline.

In the second part of an interview with television gossip show “The Insider,” Yeater responded to reports about Bieber’s statements and stood by her story.

She acknowledged the seriousness of the matter and said there was “no questions about it” the paternity test would prove the baby was Justin’s.

She declined to answer any questions about her ex-boyfriend John Terranova, or Bieber.

Terranova’s grandmother said Yeater tried to say the baby was his, which he vehemently denied. In an ensuing argument, she allegedly slapped him three times and he filed assault charges against her.

“Everyone’s had a past and made mistakes, there’s nothing wrong with that,” Yeater said. “No one’s past should matter [toward] a future situation, or anything of that matter.”

On the “Today” show last Friday, Bieber flatly denied Yeater’s allegations amid efforts to promote his new Christmas album.

“I’d just like to say, basically, that none of those allegations are true,” he told Matt Lauer. “I know I’m going to be a target, but I’m never going to be a victim.”

“I think it’s crazy because every night after the show, I’ve gone right from the stage right to my car,” he explained.

“So it’s crazy that some people want to make up some false allegations, but to set the record straight, none of it is true. Never met the woman.”