The X Factor judge confessed, “She is going to hate me for this because she is all posh in a country house but it wasn’t a ‘thing’, it just happened, we just giggled at it and that was it.
“We were best friends. Have you ever done that?”
She then added of Geri: “She had great boobs.”
Piers, who appeared shocked by the sexy admission, gushed: “This is a Spice Girls bombshell.”
He then continued, “You have never admitted this before, have you? People are going to be fascinated.”
The mum-of-three replied, “Yeah and I’ve said it now. She is going to kill me and so is her husband."
Apparently Mel made a panicked phone call to apologise to Geri straight after the show.
A source said: “The girls all know Mel struggles to keep her mouth shut but Mel C who was in the audience was visibly horrified when it all came out.
“The tour is just two months’ away they don’t want anything jeopardising it, but there were real fears this would upset Geri’s husband and causes serious issues.
Mel later added: “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.”
Geri, who is yet to respond to her bandmate’s confession, has spoken openly about her sexual experiences with women before.