(Cover) - EN Showbiz - Katy Perry was "excited" to enjoy a night of partying with Justin Bieber and Jessie J on Saturday.

The singer was in the UK to perform at Capital FM's Summertime Ball alongside her showbiz peers and decided to end the evening with a spot of clubbing in London.

The Part of Me star hit the tiles at trendy nightspot Cirque du Soir, where she spent time mingling with Justin, Jessie and her rumoured beau Robert Ackroyd.

However, the brunette beauty seemed to enjoy her night out a tad too much, as she reportedly stumbled to the floor in the early hours.

"Katy was in high spirits and very excited that Justin was there, as she loves Justin. And she's good friends with Jessie, so they had a catch-up," a source told British newspaper The Mirror. "Before long Katy had sunk a fair few vodka cocktails. She had big bottles of Ciroc and Belvedere on her table. By 5am she seemed to be having some difficulty walking and suddenly fell to the floor. She was down for a while and it took a couple of her mates to get her up. Everyone was staring and there was quite a commotion."

Meanwhile, Justin's night out ended with the 18-year-old having to change his London hotel to escape his manic fans.

His enthusiastic followers had found out that he was staying at the posh establishment The Langham and kept phoning, causing the lines to crash. According to reports, the fans even got hold of the teenage heartthrob's direct line and kept bombarding him with calls.

Justin decided it was best to move to the equally fancy The Dorchester, where he finally settled. (C) Cover Media