Nick Carter has been sued for sexual battery over an alleged incident involving a minor in 2001.
The Backstreet Boys member invited 17-year-old Shannon Ruth on to his tour bus after a gig in Tacoma, Washington, according to the lawsuit.
Ruth - who has autism and cerebral palsy - said she was offered 'VIP Juice', which she believes was a mixture of cranberry juice and alcohol.
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The suit claims Carter - who was 21 at the time - led Ruth to the toilet and ordered her to perform oral sex on him as she cried.
Ruth, who was a virgin, alleges that she was then pushed on to a bed where Carter penetrated and sexually assaulted her, calling her a 'retarded little b**ch' and telling her no one would believe her if she spoke out.
The court papers also claim that Ruth contracted sexually-transmitted infection HPV from the encounter.
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According to Variety, Ruth - who is now 39 - told a press conference: "The last 21 years have been filled with pain, confusion, frustration, shame, and self-harm that are a direct result of Nick Carter raping me.
"Even though I'm autistic and live with cerebral palsy, I believe nothing has affected me more … than what Nick Carter did to me."
Carter, 42, has not directly addressed the allegations, but his lawyer Michael Holtz told NBC the claims were 'entirely untrue'.
He said: "This claim about an incident that supposedly took place more than 20 years ago is not only legally meritless but also entirely untrue.
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"Unfortunately for several years now, Ms Ruth has been manipulated into making false allegations about Nick, and those allegations have changed repeatedly and materially over time.
"No one should be fooled by a press stunt orchestrated by an opportunistic lawyer.
"There is nothing to this claim whatsoever, which we have no doubt the courts will quickly realise."
Ruth is seeking more than $30,000 (£24,000) in compensatory and punitive damages.