The Wanted joined Justin Bieber at the weekend to switch on Christmas lights in the capital.
The major event, at the massive Westfield complex in Shepherd's Bush, saw the American teen attract huge crowds as he performed and posed for photos.
The Brit boyband joined him on stage, with Bieber tweeting afterwards: "had my boys @thewantedmusic surprise the crowd 2nite (sic)", before encouraging his 14 million followers to follow each individual member of the group.
Tom Parker responded: "pleasure was all ours bro. See you soon".
Jay told Entertainmentwise that they were excited about seeing Bieber again but that the screaming girls had been a bit too much for their security.
He said: "It was great to see him, every time we do he's about a foot taller! It was great that he played 'Lightning', and the fans we're so crazy it made our security Big Kev's ears ring cause he's used to a slightly lower pitched scream.
"Bieber rocked it and got everyone ready for Christmas. And we'll be buying Big Kev earplugs for next year!"
But Justin was unlikely to have left Tom starstruck. His bandmate Nathan Sykes told us in an exclusive interview last week that he's more gobsmacked by a different league of superstar.
He revealed: "We were in a bar last night, and Tom came round to me with the biggest kind of grin on his face, and I went 'What?' and he went 'that's f*****g Claire Sweeney over there!' which was absolutely hilarious."
The boys were also snapped up by Britney Spears, who invited them to support her at one of her UK tour dates.
Nathan added: "She just literally asked us! We went along to her gig at Wembley, and her manager showed Britney our music and asked if we should support her next week and she was like 'Yeah, go for it!'."
Justin Bieber and The Wanted at Shepherd's Bush
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