The 24-year-old, who was the only surviving original member of the band, will remain with Island Records as a solo artist.
The group have been the subject of tabloid speculation after Amelle Berrabah reportedly went missing two weeks ago, with reports claiming that Buchanan had been bullying the singer.
Berrabah has since denied the allegations; however media interest intensified following reports that Ewen had flown over to join the group.
An official statement reads: "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."
Sugababes rose to fame with debut single 'Overload' in 2000. Siobhan Donaghy was the first to leave the group in 2001 and was replaced by former Atomic Kitten singer Heidi Range.
Amelle Berrabah joined the group in December 2005 following the depature of Mutya Buena.
The group have enjoyed 24 Top 40 singles, six albums and greatest hits Overloaded - The Singles Collection.
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