The song, which is called 'Shadows' and was penned by OneRepublic's Ryan Tedder and Backstreet Boys star AJ McLean, was snapped up by Simon Cowell for the Irish boyband.
"Ryan and I wrote the song for the new Backstreet album, but it didn't make it," McLean told the Daily Star.
"When Simon heard it he loved it, and initially wanted it to go on the Leona record. Then he decided as it was more of a boyband song it'd suit Westlife better."
McLean insisted that his own group, who paired up with producer RedOne on new album This Is Us, deserve to be given more credibility.
"Some people wanna put you in a box of what was, and not really grow with you. All we want to do is make great music that we are happy with," he said.
"When you stick a brand name like BSB on a song, sometimes you put a limit on it because people think it’s going to be boyband crap."
Westlife release new single 'What About Now' on October 26. Their new album follows on November 30.