The `Part of Me` singer - whose divorce from comedian Russell Brand was finalised in July - is said to have told the `Half of my Heart` singer to lose her number because he cannot commit.
A source told US gossip website RadarOnline: "John just can`t commit, and he kept luring Katy back with promises that was going to change, but then he didn`t. Katy was pretty crazy about him, so she kept giving him another chance.
"John liked Katy, so he wanted to keep her in his life, but it just wasn`t up to Katy`s standards. She needs the guy to be on board 100 percent and she was sick of John disappearing for five days at a time, then booty calling her, then straightening up and treating her well, only to go back to his old behaviour a few days later.
"Finally she was like `lose my number`."
The on-again/off-again couple first began dating in early August but split after just two weeks with friends saying it was only a casual rebound romance following her divorce from Russell. The pair reconciled a few weeks later but this time they have split for good, according to reports. Katy is preparing to host a divorce party later this month on what would have been her second wedding anniversary on October 23.
Katy and Russell married in a romantic ceremony in India after a year of dating but 14-months later they announced their marriage was over.
A source said: "Katy has been dreading October 23 for weeks now and has been trying to think of ways to turn it into something positive."