Katy 'upset with Rihanna'

Katy Perry is allegedly "shocked and hurt" by Rihanna's rumoured collaboration with her ex-boyfriend John Mayer.

The Birthday singer broke up with John in February after an on-off relationship which began in summer 2012. The courtship reportedly ended after Katy found messages from another girl on the infamous lothario's phone. The 29-year-old is said to be upset that her supposed close friend has been spending time with the man who broke her heart.

"Katy's been left shocked and hurt after finding out John and Rihanna met up at a recording studio in LA two weeks ago before having dinner," a source told British magazine Now. "Rihanna asked John to sing vocals on a new track, but Katy's flipped out and told her best mate she's gone one step too far this time. Katy's totally unimpressed."

Rihanna reportedly shared a cosy dinner with John at LA's Soho House to discuss the deal.

Katy has been close friends with the 26-year-old since 2009 when Rihanna sent the Dark Horse singer a Valentino handbag as a present. Rihanna helped Katy through her divorce from British comedian Russell Brand but Katy doesn't feel she has been as considerate during her most recent relationship woes.

"She was madly in love with John and heartbroken when it didn't work out," the source continued. "The last thing any girl wants is to see her best friend getting chummy with their ex. It's a complete slap in the face."

The only other time the two singers have seemingly fallen out was when Rihanna resumed her relationship with ex-love Chris Brown last year. Katy reportedly disapproved but the pair soon made up and were photographed partying together last August with Rihanna captioning Instagram pictures with '#KatyAnna #RIHunited. (sic)."