Katy Perry says she sang in the background of Britney Spears' "Smurfs 2" song "Ooh La La!"

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The single was written by Perry's frequent collaborator Bonnie McKee. It can be heard playing over the end credits of the "Smurfs 2" family movie.

"Me and Bonnie secretly did some background vocals on that song," Perry told E! News. "So maybe listen for our voices."