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'?"
Spice Girls through the years
Spice Girls through the years
Victoria Beckham, Mel C, Emma Bunton, Geri Halliwell and Mel B from The Spice Girls (Photo by Jeff Kravitz/FilmMagic)
Geri Halliwell and Victoria Beckham pose for a photo shoot at the Four Seasons Hotel on April 29, 1997 in Kuta, Bali, Indonesia. (Photo by John Stanton/WireImage)
UNITED STATES - SEPTEMBER 04: The Spice Girls attending the 1997 MTV Video Music Awards, NYC. 09/04/1997 (Photo by Vinnie Zuffante/Getty Images)
LONDON, UNITED KINGDOM - OCTOBER 16: A young girl holds the Spice Girls dolls which were unveiled at the Toy Retailers Fair in London by the Character Group which has exclusive rights to distribute the dolls in the UK 16 October. (Photo credit should read JOHNNY EGGITT/AFP/Getty Images)
Paris, FRANCE: (FILES) File photo dated 16 December 1997 shows the Spice Girls posing at a hotel in Paris. Pop group the Spice Girls announced 28 June 2007 they are to reform for a world tour, saying 'girl power is back and stronger than ever,' in a statement posted on their website. AFP PHOTO THOMAS COEX (Photo credit should read THOMAS COEX/AFP/Getty Images)
INDIANAPOLIS, UNITED STATES - 1998: L to R Melanie Brown, Melanie Chisholm, Emma Bunton and Victoria Adams from the Spice Girls in Indianapolis celebrating their hit 'Viva Forever' reaching number 1. (Photo by Dave Hogan/Getty Images)
L-R Mel C (Melanie Chisolm), Geri Halliwell, Emma Bunton, Mel B (Melanie Brown), Victoria Adams of the Spice Girls pose for a studio group portrait in 1998 in Birmingham, United Kingdom. (Photo by Mike Prior/Redferns)
Portrait of UK pop group The Spice Girls (Victoria Beckham, Melanie Brown, Emma Bunton, Melanie Chisholm and Geri Halliwell) photographed in London in 1998. Job: 34206 Ref: SDD - Non-Exclusive - World Rights Only (Photo by Steve Brown/Photoshot/Getty Images)
Melanie Chisholm, Geri Halliwell, Victoria Beckham, Emma Bunton, and Melanie Brown of the Spice Girls (Photo by Jim Smeal/WireImage)
NEW YORK CITY - JANUARY 14: Pop group Spice Girls: 'Posh Spice' Victoria Adams, 'Scary Spice' Melanie Brown, 'Baby Spice' Emma Bunton, 'Ginger Spice' Geri Hallliwell and 'Sporty Spice' Melanie Chisholm promote their new movie 'Spice World' on January 14, 1998 at Planet Hollywood in New York City. (Photo by Ron Galella, Ltd./WireImage)
UNITED KINGDOM - FEBRUARY 10: BRIT AWARDS, LONDON ARENA Photo of SPICE GIRLS, Group portrait L-R Melanie Brown (Mel B), Victoria Adams (Beckham), Geri Halliwell, Emma Bunton and Melanie Chisholm (Mel C) (Photo by JMEnternational/Redferns)
UNITED KINGDOM - FEBRUARY 10: BRIT AWARDS, EARLS COURT Photo of SPICE GIRLS, Group performing live on stage L-R Melanie Chisholm (Mel C), Geri Halliwell, Victoria Adams (Beckham), Emma Bunton and Melanie Brown (Mel B) (Photo by Mick Hutson/Redferns)
UNITED KINGDOM - APRIL 01: WEMBLEY ARENA Photo of SPICE GIRLS, Group performing live on stage Spiceworld tour L-R Melanie Chisholm (Mel C), Geri Halliwell, Victoria Adams (Beckham), Emma Bunton and Melanie Brown (Mel B) (Photo by Mick Hutson/Redferns)
LONDON, UNITED KINGDOM: The Spice Girls pop group pose for photographs next to their life-size wax models at Madame Tussaud's Rock Circus in Piccadilly 17 December 1999 in London. (From L) Scary, Posh, Sporty and Baby are the first pop band combo to be modelled since the Beatles in 1964. (Photo credit should read ADRIAN DENNIS/AFP/Getty Images)
LONDON - DECEMBER 15: Sarah Ferguson with daughters Eugene and Beatrice pose with The Spice Girls on December 15, 1999 at Earls Court, London. (Photo by Dave Hogan/Getty Images)
Spice Girls in New York City, New York (Photo by RJ Capak/WireImage)
UNITED KINGDOM - DECEMBER 13: HEATHROW AIRPORT Photo of SPICE GIRLS and Victoria BECKHAM and Geri HALLIWELL and Mel B and Melanie BROWN and Emma BUNTON and Mel C and Melanie CHISOLM, L-R: Victoria Beckham, Geri Halliwell, Mel B (Melanie Brown), Emma Bunton, Mel C (Melanie Chisolm) posed, group shot, at opening of Virgin Terminal 5 (Photo by Stuart Mostyn/Redferns)
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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" 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."