Brown made the shock revelation on the upcoming series of Piers Morgan’s Life Stories, and Morgan played the clip on Monday's Good Morning Britain.
Melanie Chisholm, who was in the audience, was reportedly left stunned as Brown told Morgan of the alleged tryst with Ginger Spice, who is now married to F1 boss Christian Horner.
Morgan had asked: “You were coming here to be brutally honest - did or didn't you sleep with Geri Halliwell? Did you sleep with her?”
Brown replied that they had “all slept in a bed together” but said that is was “not like that”. She then smiled and nodded when Morgan asked if Horner was “like that”.
Brown then said: “She's going to hate me for this because she's all posh in her country house with her husband. But it’s the past.”
After seeing the clip, Morgan’s Good Morning Britain co-host Susanna Reid said: “She couldn't help herself. She still could have said: 'I'm not going to talk about it’.”
Morgan confessed that he “loved” seeing Chisholm’s face in the audience, revealing that she was “running about wondering what Brown had done” after the show.
The broadcaster also claimed that Brown rushed off set to call Horner to “try smooth things over”.
Horner has reportedly denied the claims but a past recording of her on the Howard Stern show over in America has resurfaced, showing her admitting to “having lesbian sex once”.
Horner said: “I don’t mind boobs but the other bit is not my cup of tea. I didn’t like the lesbian thing. It is part of experimenting. I was drunk at the time.”
Good Morning Britain airs weekdays from 6am on ITV. Life Stories will air later this year.