Los Angeles:
The 27-year-old singer went to a late-night house party with new man, singer John Mayer, 34, recently.
John has previously dated Jennifer Aniston, 43, Cameron Diaz, 39, Jennifer Love Hewitt, 32, Kim Kardashian, 31, and Taylor Swift, 22, amongst others.
A source says most of Katy's inner circle, as well as ex-husband Russell Brand, 36, are worried about her choice of partner.
An insider told the Daily Star newspaper: "John has a big reputation as a serial womaniser. If people thought her ex, Russell Brand, was bad, John is notoriously a lot worse.
"Katy's friends are worried this is just a rebound bit of fun but will end with her getting really hurt."
John - who also dated Jessica Simpson, 32, in between her relationships with Nick Lachey and Eric Johnson - famously described his time with Jessica as similar to taking drugs.
He said: "That girl is like crack cocaine to me.
"Sexually it was crazy. That's all I'll say. It was like napalm, sexual napalm. Did you ever say: 'I want to quit my life and just f**king snort you?'"
Read: Katy Perry's un-glamorous pre-concert rituals
John has previously dated Jennifer Aniston, 43, Cameron Diaz, 39, Jennifer Love Hewitt, 32, Kim Kardashian, 31, and Taylor Swift, 22, amongst others.
A source says most of Katy's inner circle, as well as ex-husband Russell Brand, 36, are worried about her choice of partner.
An insider told the Daily Star newspaper: "John has a big reputation as a serial womaniser. If people thought her ex, Russell Brand, was bad, John is notoriously a lot worse.
"Katy's friends are worried this is just a rebound bit of fun but will end with her getting really hurt."
John - who also dated Jessica Simpson, 32, in between her relationships with Nick Lachey and Eric Johnson - famously described his time with Jessica as similar to taking drugs.
He said: "That girl is like crack cocaine to me.
"Sexually it was crazy. That's all I'll say. It was like napalm, sexual napalm. Did you ever say: 'I want to quit my life and just f**king snort you?'"
Read: Katy Perry's un-glamorous pre-concert rituals
