The `Toxic` singer is part of the new line up on the US version of the singing talent show, and doesn`t give budding singers an easy time if their renditions of songs aren`t up to scratch.
A source told People.com: "She`s got a really good instinct, and when she loves something, she looks like she wants to dance on the desk. "When she doesn`t like someone, she is incredibly tough."
Britney`s harder side was revealed at the show`s auditions in Kansas at the weekend, where she starred on the panel alongside Demi Lovato, L.A. Reid and Louis Walsh, who stepped in from the UK version of the show to replace Simon Cowell, who was taken ill with bronchitis.
The source added: "You have to be great to be on this show, so I vote no."
Britney is not afraid to stick up for contestants she likes, either, and the show is said to be a good boost for her own confidence.
The source said: "She`s really massively grown in confidence. She loves interesting and fun auditions and is prepared to stick her neck out and support someone no matter if some of the other judges disagree."
The next round of auditions for `The X Factor` take place in San Francisco on June 16, and Simon is hoping to have recovered in time to take part.