Britney Spears has serenaded L.A. Reid and surprised him with a birthday cake on an episode of 'The X Factor' USA.

The 'Toxic' hitmaker sang 'Happy Birthday' to her fellow talent show judge - who turned 56 on June 7 - with her fiance Jason Trawick, after the pair presented the music mogul with a cake and candles backstage in the talent show's green room during the auditions stage of the competition in Kansas City.

In footage aired on the programme last night (19.09.12), L.A. said: ''They don't have enough candles for my age.''

He then told the 30-year-old singer: ''You're the sweetest in the world, oh my god, thank you.''

The Kansas auditions were held one day after the talent show judge - who sits alongside Simon Cowell and Demi Lovato on the panel - turned 56, but Simon was unable to celebrate L.A.'s birthday in June as he was ill at the time meaning Irish music mogul Louis Walsh had to stand-in for him.

Britney has been no stranger to organising birthday celebrations recently after throwing a huge joint birthday party on Saturday (15.09.12) for her two sons, Jayden James, who turned six last Wednesday (12.09.12) and Sean Preston, who was seven on Friday (14.09.12).

The blonde beauty hired a Sky High arcade and trampoline park in Calabasas, California, to celebrate her sons' birthdays, and she invited 20 of the youngsters' friends to party as well as Jason and her father Jamie Spears. But it's not known if the children's father, Kevin Federline, attended the bash.