Britney Spears with L.A. Reid, Demi Lovato and Simon Cowell

Britney Spears is a "fearless" judge on US 'X Factor'.

The pop star joined the panel of the show for its second season alongside fellow newcomer Demi Lovato and original judges Simon Cowell and L.A. Reid and Simon was hugely impressed with the 'Toxic' singer.

He said: "She's fearless. When she says 'no' or disagrees, she couldn't care less.

"I think she's really enjoying doing something other than singing. I think she likes being part of a team."

However, he admitted it wasn't smooth sailing from the start, explaining: "The first day, the second she said 'no,' everyone booed her and that happened a lot and I think she found it very intimidating."

Yet Britney thinks Demi is even tougher than her, calling the 20-year-old judge a "spitfire".

She said: "I think Demi [is tougher], I just do. She's a little younger than me so she's a spitfire."

Meanwhile, Demi admitted she and Britney, 30, struggled at the start of the competition.

She told People: "We got more comfortable in telling our opinions without being afraid of the booing. When Britney and I first started on this show, we were looking at each other like, "What do we do? We're so nervous!'"