According to the biography, "Lily Allen: Living Dangerously", Lily Allen was very nearly a fourth member of the original line-up of Sugababes.
In the late 1990's, All Saints manager Rom Tom heard a demo recording by Lily Allen and liked her voice. At the same time, he discovered two young girls by the names of Mutya Buena and Siobhan Donaghy. He was later introduced to Buena's best friend Keisha Buchanan and he decided to bring the girls together as a trio to form the Sugababes. Soon after, they were signed to London Records. The record label initially wanted to bring in a fourth member, so the manager contacted Lily Allen and asked her if she'd be interested in joining the band. She considered it for a while, but later declined. Two years later she was signed to London Records as a solo artist, but was later dropped. In 2005 she signed to EMI's Regal Records after she became the first musical phenomenon on social media sites like MySpace.
This has also been confirmed by a moderator on Lily's former official website forum.
Thank God she declined, we would have never had her flawless discography
We probably would as she would have definitely left the Sugababes early on like Siobhan did. Those girls were too feisty with each other in the early days.
