The supposed admission came up while Mel, aka Scary Spice, was being interviewed by Piers Morgan for his new ITV show Life Stories on Friday.
"Did you or didn't you with Geri Halliwell...did you sleep with her?" Morgan asked, according to Mail on Sunday, referring to long-standing rumors involving the two women.
"Yeah, we all slept in a bed together but not 'like that,' all of us," she answered, seemingly attempting to skirt the question.
But Morgan persisted, asking her to clarify: "Did you sleep with Geri 'like that'?"
That’s when the 43-year-old singer offered a smile and a nod, stating, "She's going to hate me for this because she's all posh in her country house and her husband. But it's a fact. It just happened and we just giggled at it and that was it."
Mel followed that up by explaining that it was not a blossoming romance, just a sexual tryst between the fellow singers.
"It was just that once. And hopefully when Geri gets asked that, which hopefully she will after this, she won't deny it,” she said. “Because it was just a fun thing."
However, a source tells ET that Mel's claims are “untrue,” adding that the supposed admission was “just Mel B being Mel B.”
Halliwell has been married to Christian Horner, the head of a Formula 1 racing team, since 2015. Mel was married to film producer Stephen Belafonte from 2007 to 2017.
According to Mail on Sunday, Melanie Chisholm -- aka Mel C -- was in attendance for the interview and Morgan asked her if she was aware of this supposed sexual encounter, to which she replied, "I don't know anything."
"Oh whatever, Mel," Mel B told her.
"This is all new to me," Mel C said, defending herself.
The Sun on Sunday also reports that Morgan asked Mel B if she and Mel C had ever done "anything," and she replied, "Well, when I got my tongue pierced we all kissed. But that was just a kiss because I wanted to know what it felt like. It was just a silly kiss. Nothing sexual like that."