Melanie Brown has claimed she and Geri Horner slept with each other during the height of Spice Girls fame.
Brown, 43, said the pair slept together during the group's heyday and "giggled" about it after.
She said Horner, 46, would "kill her" for revealing the reported encounter, according to The Sun.
Brown made the claim to Piers Morgan on Friday during the recording of an episode of his Life Stories programme.
Brown - known during her time in the group as Mel B - reportedly nodded when asked by the Good Morning Britain presenter, "Did you sleep with Geri?"
The singer, who is bisexual, also complimented Horner's "great boobs".
She is reported to have said: "(Geri) is going to hate me for this because she is 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. We were best friends. Have you ever done that?"
Pressed on how many times they had slept together, Brown is reported to have said it had happened just once.
Morgan added: "This is a bombshell. A Spice Girls bombshell. You have never admitted this before have you? People are going to be fascinated."
She replied: "Yeah, and I have said it now. All done. She is going to kill me and so is her husband."
Horner is married to Christian Horner, team principal of the Red Bull Formula One racing team.
She continued: "Hopefully when Geri gets asked that, which hopefully she will after this, she won't deny it, because it was just a thing, a fun thing."
Her bandmate Melanie Chisholm - known as Mel C - was in the audience as the ITV show was recorded. When pressed by Morgan, she said: "This is all new to me."
Emma Bunton, Chisholm, Horner and Brown will reunite as the Spice Girls for a stadium tour, which kicks off in May in Ireland, although Victoria Beckham is not taking part.
Reporting by Press Association