Appearing on US chat show Larry King Live, 39-year-old Mel said she still reminds Geri of the betrayal she felt over her exit.
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She said: "When Geri left the group, it was bad. She left on my birthday, and didn't tell anybody… she just didn't show up. We had the rest of the tour. The American tour."
While she's over it now, Mel admitted to feeling bitter 'for a long time', adding: "I still remind her of it. Like, I saw her a couple of weeks ago and I said, 'Do you remember when you left on my birthday?'"
But Mel did say she understood why Geri had to get out. Following her exit, Geri admitted to having an eating disorder and Mel added: "She just felt the pressure too much, so just decided to take a break."
During the interview Mel also revealed that infighting led to the group splitting up before planned.
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She said: "We fought like cats and dogs. We had a five-year plan that got shortened after three years."
But Mel and the others, Victoria Beckham, Melanie C and Emma Bunton, always 'knew this was a platform'.
She added: "We always knew Victoria was going into fashion, Mel C was going into music, Emma went into radio, and I wanted to do a bit of everything."