Simon Cowell has described new The X-Factor USA judges Britney Spears and Demi Lovato as the ‘gruesome twosome.’

The media mogul, who this week dismissed reports that Britney has become unstable due to the show’s demands, took to Twitter to poke fun at his new recruits after finishing a round of auditions in San Francisco.

He said: “Unfortunately Britney & Demi are now known as the gruesome twosome.V happy to be back-Demi more annoying than ever, Britney has grown claws!”

After retweeting the message, Britney then replied: “And I’m not afraid to use them.”

The ‘Womanizer’ star had previously teased her new boss on the social networking site ahead of the day’s filming, posting:

“I'm glad @SimonCowell is feeling better…I know @ddlovato, @LA_Reid & I aren’t cutting him any slack today! #SFactor”