The 'Flying Without Wings' band left Cowell's label after 13 years back in March and signed a new multi-million pound deal with Sony RCA. They have now admitted that the reason for their departure was down to restrictive single release schedules.
"Simon's company has become more about TV as the music industry is shrinking," band member Kian Egan told the Daily Star.
"We'd have arguments about wanting to release more than one single with Syco, and Simon wouldn't even be involved. Leaving was the best decision for us."
The band are now preparing to release a new single along with a greatest hits compilation.
"The new single is up-tempo and very different for Westlife. Mark Feehily co-wrote it with John Shanks," Egan added.
No release date for the new material has yet been confirmed.
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Watch the music video for Westlife's hit 'Flying Without Wings' below: