Katy Perry reportedly went out of her way to show Rihanna she has "replaced" her as a friend.
Rihanna recently confirmed her romance with Chris Brown is back on, and this is said to have caused a breakdown in her friendship with Perry.
The two women were very close, with Rihanna even planning Katy's bachelorette party before her wedding to Russell Brand.
Chris and Rihanna split after he assaulted her in 2009 and she has insisted he has changed. Perry reportedly can't understand why her pal is giving things another go.
The two female stars didn't sit together at the recent Grammy Awards, with Perry accused of sending a message to the Barbadian beauty when she spent time with Girls star Allison Williams at an after-party.
"It's almost as if Katy was showing Rihanna that she's been replaced, as they usually spent time catching up at these events. Instead, she chose to hang out with Allison," an insider told Heat magazine.
Perry has reportedly branded Rihanna's romance "toxic" and thinks she and Brown only bring out "the worst" in each other.
It's rumoured that Rihanna was never sure of Perry's relationship with Russell, but was there to support her friend during her divorce.
Now Perry is dating former womaniser John Mayer and Rihanna thinks it's time her friend butted out of her personal life.
"Rihanna's attitude is that she's tired of everyone having an opinion on her life - including Katy," the source said. "Rihanna doesn't think that Katy should be judging her relationship with Chris when she's involved with John, who is a well-known player."