The blonde singer is alleged to have secretly made a deal with bandmate Amelle Berrabah to push out Keisha - the last remaining founding member of the group - because she thought Keisha would ruin their chances of success in the US.
A source told Britain's The Sun newspaper: "Keisha clashed with Amelle since she joined.
"Heidi did not want to be in a position where a big bust-up might arise while they're promoting their material in America.
"So she persuaded management a less headstrong character would be better suited to the group."
The US remains a crucial market for the Sugababes to crack and they have recently secured a multi-million deal with Jay-Z's new label Roc Nation.
Keisha has been replaced with former Eurovision Song Contest Jade Ewen - who has promptly been sent into the studio to re-record all of Keisha's vocals for forthcoming album 'Sweet 7', due for release on November 23.
Keisha, 24, was seen in tears at the Red Bull Fashion Factory party, which was part of London Fashion Week, on Monday night (22.09.09), just hours after she announced she had left the girl group.
She was comforted in the VIP section by British boy band JLS and former Destiny's Child singer Michelle Williams.
A source said: "Michelle was the first to offer words of encouragement. But it all got too much for Keisha, who started to cry in the middle of the party. She was overheard saying, 'I'm devastated about what happened. Everyone hates me. I did everything I could to make it work between me and the girls. I'm so lost."
The Sugababes have a history of bust-ups within the band and have seen several line-up changes.