It had been suggested that the trio could reform the group's original lineup, though record label Warners denied approaching them about a comeback.
Buchanan's manager told BBC Newsbeat: "Keisha is definitely, 100% not going to be involved in any kind of Sugababes reformation.
"Keisha has solo projects that she's working on which will be out next year."
Buchanan had previously used Twitter to deny reports of a reunion, while Buena was said to be keen on the mooted comeback.
The current Sugababes lineup features Heidi Range, Amelle Berrabah and new addition Jade Ewen, none of whom were in the group when it was formed in 1998.