Katy Perry and John Mayer's relationship can only end "in disaster", it has been claimed.

The pop star is said to have fallen head-over-heels for the renowned ladies man after failing to keep things casual between them.

Friends fear that John could drop Katy at any minute, just like he did at the beginning of their courtship when he ditched her after just a few weeks of dating.

"Despite the fiery image Katy likes to present, she's actually more fragile than that, and when John dumped her a few weeks after they first got together she was devastated," a source told Radar Online. "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 27-year-old star's nearest and dearest are reportedly concerned about how quickly Katy's got involved with the 34-year-old singer. The songstress has apparently been gushing to anyone who will listen about how much she loves him, but those closest to her find this uneasy information to hear.

"Katy's friends and family haven't taken to John at all, in fact they are worried that she is headed for heartbreak," the source revealed. "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."