The 'Shake It Off' singer ended her relationship with the Scottish DJ early last month and the ''breaking point'' in their 15-month romance reportedly came after he was ''disrespectful'' by saying he'd never work with her while promoting the track.
According to TMZ, Taylor wrote the song and demoed it on her iPhone, sending the file to Calvin. They then went into the studio to make a fuller version, featuring the 26-year-old star on vocals and the 32-year-old producer doing the beat.
Though they knew the track would be a hit, the pair felt their relationship would overshadow the song itself, so Calvin brought in Rihanna to record the vocals, and Taylor - who kept the publishing rights - was credited under the pseudonym Nils Sjoberg.
And much to Taylor's upset, on the day the song was released in April, Calvin said he ''couldn't see'' himself ever working with his then-girlfriend.
He told DJ Ryan Seacrest: ''You know we haven't even spoken about it. I can't see it happening though.''
A source explained the interview left Taylor upset and marked the start of a downward spiral for the couple.
They said: ''Taylor was hurt and felt Calvin took it too far'',
After splitting from the 'Summer' hitmaker, the 'Blank Space' singer is now dating Tom Hiddleston, much to Calvin's fury, and her friends have now decided to speak out about the song because they think the DJ is being unfair to her.
TMZ explained: ''[Her friends are] sick of Calvin trashing Taylor and feel he should be thanking her for quietly giving him a smash hit.''
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