Pop star Justin Timberlake shocked TV host Ellen Degeneres by calling her up during her show to wish her a happy birthday.

The talk show star celebrates her 50th birthday on Saturday, and was taken aback when the SexyBack star phoned her on Wednesday - and pretended to be fictional English nanny Mary Poppins.

DeGeneres looked visibly confused upon hearing the high-pitched English accent on the other end of the phone - and was still unsure of the caller's identity even when he reverted to his normal voice.

He told the star, "I've been on your show a couple of times and I really think you're really sexy," before admitting, "It's hard for me to break into numerous voices - I get nervous and sweaty when I talk to you anyway."

Timberlake then revealed his identity to an excited DeGeneres, who congratulated her pal on his five Grammy nominations and arranged to meet up to play music-based video game Guitar Hero.

The singer - who was phoning from Miami - ended the call wishing DeGeneres best regards on her upcoming landmark celebration: "I called to say happy 70th (and) to tell you you look so freakin' hot. And remember, Ellen, 50 is the new 60."