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Scary Spice made the revelation while sitting down with Piers Morgan for an episode of Piers' Life Stories, which is set to air in two months time, and a clip of the confession was previewed on today's (Monday, 25th March) Good Morning Britain.
"You were coming here to be brutally honest - did or didn't you sleep with Geri Halliwell? Did you sleep with her?" questioned the notoriously outspoken journalist in the clip.
To which Scary Spice replied: "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."
Shedding a little more light on their rendezvous, Mel also added: "We were best friends. It just happened."
And it seems like they kept their fling completely secret as Sporty Spice aka Mel C, who was sitting in the audience, declared: "This is all new to me."
Mel B then added: "I have said it now. All done. She is going to kill me."
Geri is yet to comment on the revelation, although reports suggest she will release a statement to set the record straight.
The Spice Girls are going on tour later this year - without Posh Spice - but Mel's interview is said to have ruffled some feathers among the group.
According to Piers, Geri is worried about what her husband, F1 chief Chris Horner, will make of it all.
He said: "Geri is now briefing the 'it's never happened'. She's slightly panicking about what Mr Geri [husband Christian Horner] thinks."
Geri and Chris tied the knot in May 2015 and share a son together, Montague, two. Geri has a 12-year-old daughter, Bluebell, from a previous relationship.
We're guessing this won't be the last we hear about this rumoured fling.