Taylor Swift Vs. Katy Perry: Swift Fans Boo Perry at British Summertime Festival -- Details Here

The rivalry between Taylor Swift and Katy Perry continues! During a music festival in the United Kingdom over the weekend, Katy Perry reportedly received a ton of heckling from fans of Taylor Swift. The "Firework" singer wasn't even in the event, but her image was flashed on the large screens, according to Hollywood Life.

The two pop stars were part of a video montage meant to air at the British Summertime Festival in London; and while Taylor's air time was met with cheers and hoots, Katy's appearance in the clip received a lot of boos.

As everyone waited for the concert to start, a 17-minute video featuring several artists, including Swift, Perry, Ed Sheeran, One Direction and Sam Smith played on the jumbotron. It is the same video that was used by the organizers to promote the festival on British networks.

When Katy's face first went up on screen, boos were heard from the crowd, particularly from the Swifties who came to watch their idol open the festival. A few seconds after that, Katy's face popped on the screen again and the boos got even louder. In contrast, when Taylor's face appeared in the monitor, the crowd went wild. They cheered for Taylor.

Taylor and Katy used to be friends and even said their praises towards each other in public, according to Digital Spy. Their falling out, however, had one common denominator: John Mayer. But neither Taylor nor Katy would confirm that their fight was about a guy.

Their animosity grew even bigger when Taylor released "Bad Blood," a song she reportedly wrote for Katy as a diss. She never mentioned any names, however, but said that the song was about a female singer whose relationship she sees as "straight-up enemies."

"For years, I was never sure if we were friends or not," Taylor said, according to Digital Spy. "She would come up to me at awards shows and say something and walk away, and I would think, 'Are we friends, or did she just give me the harshest insult of my life?'"

Taylor insisted that the rift was more professional in nature. "She basically tried to sabotage an entire arena tour. She tried to hire a bunch of people out from under me. And I'm surprisingly non-confrontational - you would not believe how much I hate conflict. So now I have to avoid her. It's awkward, and I don't like it," the "Shake It Off" singer revealed.