Beckham has already dismissed as ludicrous claims from Rebecca Loos that she had sex with the 28-year-old midfielder in Madrid.
Beckham and his pop star wife Victoria called in lawyers after a Sunday newspaper printed allegations from a second woman -- 29-year-old lawyer's daughter Sarah Marbeck -- who said she had an affair with Beckham.
But Loos, the 26-year-old daughter of a Dutch diplomat, insisted in a television interview being screened on Thursday night that she was telling the truth and could prove it.
"There's something I know about him, an intimate part of his body that I think only women who have been in bed with him would know," she told Sky One in the interview.
"So I think if they did want to take me to court, I could easily stand up and say 'Well' ...."
When asked if Beckham was circumcised, Loos said: "I'm not going into that. I'm not talking about that. If I do ever need to talk about that, it'll be in a court, not on TV."
A spokesman for Sky One told Reuters:
Britain's tabloid press, who have gone into a feeding frenzy over the claims, gave blanket coverage to Loos again on Thursday. Bookmakers even offered odds on what the intimate detail might be -- from tattooed buttocks to a spotty back.
Loos' latest claims were splashed over the Sun, Britain's leading tabloid which, along with Sky Television, is part of the global media empire owned by Rupert Murdoch.
In the interview, Loos shows off raunchy text messages that she said she received from Beckham, a British sporting and fashion icon famed for his squeaky clean image as a devoted family man.
She also said Beckham fed her strawberries at breakfast in a Madrid hotel bedroom and made her feel "like a million dollars".
Insisting her story was 100 percent true, she said: "There is no doubt in my mind. I have no reason to lie. Why should I? I was wrong to have done what I did. But it did happen and I'm not going to live a lie for the rest of my life."