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.
A source said: "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 withJohn 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."
Those close to Katy - who split up with Russell Brand after a short lived marriage last year - believe any potential relationship could end in "disaster" for her.
The source added to RadarOnline.com: "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!"