Miss World Singapore (MWS) 2013 finalist Vanessa Tan has been accused of being a freelancer on sex forums such as Sammyboy.
Freelancer is a commonly used label for a freelance prostitute.
Pictures resembling Miss Tan, but with her face either digitised or cut off, were featured in at least one online advertisement for a freelancer named Felicia.
A netizen also claimed on online forums that he had paid Miss Tan for sex through an agent.
The 19-year-old freelance model-dancer has now challenged the netizen to come forward to back up his allegation.
Prominent criminal lawyer Subhas Anandan, who happens to be a judge at this year's pageant, said Miss Tan is considering taking legal action against the netizen.
He said that she stands a high chance of winning the case. The 17 MWS 2013 finalists met the media at a press conference on Monday at the Pan Pacific Singapore hotel.
When asked about the allegation, Miss Tan said: "When my friend showed me the screen grab of the netizen who claimed he had sex with me, I was not shocked. There will always be people who want to bring you down.
"I want this guy to show himself and show me proof that I had sex with him. Come face me if you dare."
As for the pictures used in Felicia's online posts, Miss Tan said: "Sure, the pictures used are mine, but they were stolen from me."
According to the post, Felicia is a 1.55m tall, 20-year-old Chinese woman who weighs 40kg and has a tattoo.
The advertisement also states what services she is willing to provide to her clients. A mobile phone number was included in the earlier posts but missing in later ones.
All the posts have been deleted from the forums.
Miss Tan is 1.72m tall, weighs 48kg and has a tattoo of a butterfly on her lower back, which is apparent in some pictures on her Facebook page.
She said she has always posted all pictures from her photoshoots on her Facebook page, which is open to the public.
She added that she does bikini and lingerie shoots only for the private collections of photographers because she's proud of her figure and "doesn't want to hide it".
Miss Tan, who insisted that she would never do nude shoots, said: "I think I'm too skinny, but I love my body and I like to showcase it in a sexy and emotional way."
Event organiser Edmund Ooi of Asia Music People said that representatives sat down for a long talk with Miss Tan after they caught wind of the forum posts early last month.
Mr Ooi told The New Paper: "Vanessa told us that she had been framed. We told her not to drop out of the pageant if the accusations weren't true because it would look worse for her if she did.
"I had personally invited Subhas to be a judge this year, but that was way before the forum posts that targeted Vanessa were posted.
"He is here precisely to uphold the integrity of this pageant and educate the girls on when remarks become defamatory and if they have a case."
He added: "Subhas is here to let people know that once they cross the line, they will be punished.
"I wasn't shocked when Vanessa told me what happened. I was just relieved to have an assurance from her that it was a case of abuse."