After her involvement was revealed Taylor's name was added to the credits by BMI, which protects licenses for songwriters.
After Taylor's team confirmed that she co-wrote the song, Calvin took to Twitter to slam her for trying to 'tear [him] down'.
He wrote: 'And she sings on a little bit of it too. Amazing lyric writer and she smashed it as usual.
'Hurtful to me at this point that her and her team would go so far out of their way to try and make ME look bad at this stage though. I figure if you're happy in your new relationship you should focus on that instead of trying to tear your ex bf down for something to do.