Katy Perry is "totally in love" with John Mayer.

The "Firework" singer is worrying her friends and family by getting increasingly close to the legendary lothario - who has previously dated Jessica Simpson and Jennifer Aniston - but is falling "dangerously" towards something serious with him.

"Katy's friends and family haven't taken to John at all, in fact they are worried that she is headed for heartbreak. She's been telling everybody who cares to listen how great John is and it's clear that she is totally in love with him. At first, she was quite careful with John and said she wasn't ready for a relationship and just wanted to have fun. But now she appears to have fallen dangerously in love with him and her friends are concerned," a source said.

Those close to Katy believe any potential relationship could end in "disaster" for her. "Now that she has invested more feelings in John and has let her guard down, there's only one way this can end... in disaster!" the source added.