The 'Blank Space' hitmaker - who has infamously clashed with Katy in the past - recently split from the 32-year-old DJ before hooking up with Tom Hiddleston, and Calvin has hit out at his ''hurtful'' ex shortly after the blonde star confirmed she wrote his song 'This Is What You Came For'.
In a series of tweets, he wrote: ''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
''I know you're off tour and you need someone new to try and bury like Katy ETC but I'm not that guy, sorry. I won't allow it
''Please focus on the positive aspects of YOUR life because you've earned a great one
''God bless everyone have a beautiful day (sic)''.
Calvin also confirmed Taylor lent her vocals to part of his latest track and praised her lyrical abilities.
He tweeted along with an upside down smiley face after the report was confirmed: ''And she sings on a little bit of it too. Amazing lyric writer and she smashed it as usual (sic).''
Earlier today (13.07.16), Taylor's representative confirmed she had penned his tune, telling MailOnline: ''Taylor Swift wrote 'This Is What You Came For' under the pseudonym Nils Sjoberg.''
Calvin brought Rihanna in to record the vocals on the song as he and Taylor, 26 - who dated for 15 months - feared their relationship would overshadow the track if they released it together.
The so-called ''breaking point'' in their romance reportedly came after he was ''disrespectful'' by saying he'd never work with her while promoting the track.