Britney Spears has sung the classic song 'Happy Birthday' on X Factor USA.

The pop princess is appearing on her first X Factor series this year as a judge after signing up for the show earlier this year alongside fellow newbie Demi Lovato.

She had been filming auditions for weeks and now the series has started to air in the US.

Spears was seen singing 'Happy Birthday' on a recent programme to colleague LA Reid, who was celebrating his 56th birthday at the time.

The vocal cameo appeared on an episode in which X Factor UK stalwart Louis Walsh filled in for an absent Simon Cowell.

Britney Spears meanwhile thanked fans last night (September 19) for enjoying the 2012 X Factor USA series so far, posting on Twitter:

"I'm so happy y'all are into the show. I love it too. Tweeting along again for tomorrow's episode. Hope 2 C U 2! Night tweople :)."