The former Spice Girl said she wants to focus on being a children's author and has no plans to record any more songs.
She is quoted as saying: "I feel like I've hung up my hotpants now. I'll always try and be honest about myself. I've finished as a solo artist. Right now I just feel really comfortable writing books."
She added that the group's recent reunion tour had cemented her friendships with her bandmates, saying: "I feel we are just like a family. We clicked again straight away - it's just like a family travelling in a bubble.
"I really enjoy our relationships, but I always wish I had left the group under better circumstances, rather than just walking out. But that's exactly why I wanted to do it again."
Her first children's book Ugenia Lavender is released on May 2.
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