Melanie Chisholm said it was a difficult decision not to join the latest Spice Girls reunion - but "something didn't feel quite right and I had to follow my gut".
Geri Halliwell, Emma Bunton and Melanie Brown have been dropping big hints that the trio will reunite.
But now Sporty Spice has revealed in a piece she penned for Love magazine why she will not join her former bandmates on stage.
Mel C said she will be a "Spice Girl until I die" but endless speculation about whether the band will get back together to celebrate 20 years of their debut single Wannabe has been "exhausting".
The band reached "a peak" at the 2012 London Olympics, when they sang Spice Up Your Life on top of black cabs, she said, adding: "There's a lot to be said for bowing out on a high note."
She added: "I totally get it. But is it a new rule that bands have to reform? Why can't we just be remembered for our incredible achievements in the 90s?
"Earlier this year after several face-to-face meetings with the girls, I made the difficult decision not to be part of a proposed reunion with Emma, Geri and Melanie."
Victoria Beckham had already turned the reunion down, she said, because she was busy with her fashion label and her family.
"The hardest part for me was letting people down, the girls, the fans, civilisation?! Unfortunately something didn't feel quite right and I had to follow my gut," Mel C added.
"I'd love to play huge arenas across the world, sing our brilliantly bonkers pop songs and relive our former glory. It is of course a very lucrative opportunity too. But we were a five-piece band.
"Anything less than the full line-up just didn't feel like we'd be doing justice to the band or the fans."
She said that while there will always be "hurdles with friendship", she and her former bandmates "have a very unique bond... and I will continue to support them all as much as I can."
Mel C also criticised modern celebrity culture, saying: "Wanting to be famous just to be famous without talent?... It's like going for a burger but only eating the bun - you're missing the best part."
Emma, Geri and Mel B previously hinted at the prospect of a reunion gig in July next year in Hyde Park, releasing a video saying: "We hope to tell you soon what you want, what you really, really want."
And Emma recently admitted she wanted a Spice Girls reunion, saying: "There's nothing set in stone. I personally think we should do something. It would be lovely.
"I think we need a Posh."