Melanie Brown, who was known as Scary Spice in the Spice Girls, sat down with Morgan earlier this year but the interview was aired on Sunday (NZ Time) in the UK.
Mel also had strong words for Beckham, who skipped out on the Spice Girls reunion tour, which kicked off on Friday.
The singer made the big reveal during an appearance on Piers Morgan's Life Stories, which aired in the United Kingdom on Saturday night - a day after her return to the stage with the Spice Girls in Dublin, Ireland.
Asked if she'd had a "boob job", she replied, "Well you could say it in a nicer way!" "I'm gutted she is not going to be there, but still holding a candle that she does".
Morgan called her revelation "a Spice Girl bombshell", and news of the relationship surfaced in March.
The conversation then turned to the more serious side of Mel B's life, detailing the regular racism that she encountered as a mixed-race girl growing up in Leeds. She choked up as she talked about her dad and how she regretted cutting him out of her life before he died.
But there were messages of praise too for Melanie Brown, Geri Horner, Emma Bunton and Melanie Chisholm, reported USA Today.
As for former band mate and one-time Posh Spice, Victoria Beckham?
Sound was an issue for some at the concert, with numerous tweets attesting to the fact that the songs were almost indistinguishable from one another in same parts of the stadium.
The 46-year-old singer, who is a mother of three, thanked fans for sticking with them. She said, however, that her pregnancy was planned and that they were "madly in love" before it went "dramatically wrong".
The Spice Girls were scheduled for 12 more shows all over the United Kingdom, with Cardiff's Principality Stadium on Monday the next stop on the tour.