The singer also calls ex Eddie Murphy the love of her life.
Mel B was put on the spot during the latest installment of "Piers Morgan's Life Stories," where the Spice Girl was asked if two ever became one between her and Zac Efron.
There were rumors the two hooked up back in 2017, after they reportedly met on a dating app following singer's split from her husband Stephen Belafonte.
While Brown's interview with Morgan was filmed back in March -- and made headlines at the time when a clip of her confirming a tryst with Geri Halliwell was released -- it finally aired in full over the weekend in the UK.
At one point, she was asked about her sex life and whether the gossip about her "High School Musical" hookup had any truth to it. With raised eyebrows, she didn't say a word, leading Morgan to say, "No denial?" Her reaction: "But no comment."
Seems like an easy thing she could have squashed if it weren't true, that's all we're saying.
Mel was also asked if she defines her sexuality, as she has been intimate with both men and women. "I don't," she explained. "I don't think I have to. You fall in love with whoever you fall in love with and I just don't have a preference."
That's when Morgan asked about her history with Geri, as bandmate Melanie Chiholm sat in the audience.
"She had great boobs," said Brown. When asked if she "slept with Geri, like that," she then nodded affirmatively. "She's gonna hate me for this. She's all posh in her country house with her husband. But it's in the past. But it wasn't a thing, it just happened."
When asked how many times it happened, she shot back, "Stop it, you pervert!"
"I don't know anything," added Mel C from the peanut gallery. "This is all new to me."
Later, she was asked whether Eddie Murphy -- with whom she shares 12-year-old daughter Angel -- was the "love of your life." She responded by saying "he is and always will be."
After Morgan played an old interview clip of Murphy questioning their daughter's paternity, Brown said "it's not a nice thing to hear." She added, "I wouldn't have said that publicly and he apologizes to this day about all of that. We planned that baby together and we were madly in love. It just went wrong, dramatically wrong."
There also still seemed to be lingering feelings for him, but shut Piers down when he asked how she'd react if he ever wound up single again.
Elsewhere in the hour, Brown also sounded off on her relationships with the rest of the Spice Girls.
Her honest opinion of Geri: "I love Geri to bits, but she's batshit crazy. I adore her, I absolutely love her."
Her opinion of Victoria Beckham was pretty similar, as she called Posh -- who sat out the latest reunion -- "a bit of a bitch," but added, "I love her."