The singer insists it was not her decision to leave the trio - of which she was the only remaining founder member - but the announcement she is to be replaced by UK Eurovision entrant Jade Ewen ends days of speculation about the group's future.
Following the announcement, Keisha wrote on her Twitter page: "I've had a great time and have achieved more than I ever thought I would.
"Although it was not my choice to leave, it's time to enter a new chapter in my life. I have nothing but positive things to say about the girls and I wish them the best of luck. I would like to state that there were no arguments, bullying or anything of the sort that led to this.
"Sometimes a break down in communication and lack of trust can result in many different things. I would also like to point out that I have always supported the girls and they have also supported me.
"I want to thank the Sugababes fans and my fans for all your support and all who have never judged me, but accepted me for who I am... The feisty, funny, professional, fearless motor mouth. This is not the end... but the beginning!"
While the new Sugababes line-up are preparing to release a new album, Keisha is to embark on a solo career.
Record label Island released a statement saying: "The current line-up of the Sugababes has disbanded.
"Heidi Range and Amelle Berrabah will continue as the Sugababes and will be joined by new member Jade Ewen. They release their album 'Sweet 7' on November 23rd through Island Records.
"Keisha Buchanan will continue to record for Island Records as a solo artist."
The Sugababes have a history of bust-ups within the band and have seen several line-up changes.