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Music mogul Simon Cowell has warned viewers to watch out for Britney Spears dark side The X Factor’ 's new U.S. series.

Cowell made Spears a judge on the hit talent show earlier this year, but he was forced to defend the pop princess amid reports she walked off the set after just 20 minutes.

The Toxic hitmaker later admitted she had been suffering "panic attack after panic attack" while shooting the series – but Cowell is adamant viewers will love Spears' turn on the panel, as she's the programme's new Mrs. Nasty.

He says, "I've seen the first show and I think it's sensational. I think the girls do a great job and the contestants are great. The show looks different than anything we've ever made before.

"She is a fascinating person because you just don't know anything about her. She's very unpredictable. You never know what is going to happen. She has taken this very seriously and she's surprisingly quite mean."

And it's clear there is no bad blood between the music mogul and the pop star – they were full of praise for each other after spending Sunday together.

In a post on Twitter.com on Sunday, he writes, "I spent the day with britney. She really is quite charming", prompting Spears to respond, "Oh now you want to be nice! Hehe. U are quite charming yourself... today was fun!"