Britney Spears: X Factor Judge Sings 'Happy Birthday' To LA Reid (VIDEO)

"The X Factor USA"  judge Britney Spears joined the contestants and got in on the singing action.

On the third episode of season 3, which aired on Wednesday, Sept. 19, Spears sang "Happy Birthday" live on camera to fellow judge L.A. Reid.

In the 70-second clip, shown below, Spears surprised Reid when she entered the judges' private green room carrying a green and white birthday cake with lit candles.

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The 31-year-old sang a Marilyn Monroe-inspired rendition of the traditional birthday song with her fiancé Jason Trawick in tow.  

After Reid blew out the candles, he gave Spears and fellow judge Demi Lovato a hug and thanked them for the well wishes.

"The X Factor" creator and co-judge Simon Cowell was unable to join the trio for the heartfelt moment because he was out ill. "The X Factor UK" judge, Louis Walsh, took over Cowell's judging duties for the night.

Watch the clip below and before it ends you hear Lovato saying there is no number on the birthday cake, which drew laughs. L.A. Reid turned 56 on June 7.

"The X Factor USA" season 3 airs on Wednesdays and Thursdays at 8/7c on FOX.