Katy Perry's loved ones are worried about her latest choice of boyfriend.

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. "John has a big reputation as a serial womanizer. 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," an insider said.

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. "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?'" he said.