Robbie Williams has revealed that Sharon Osbourne once mixed him up with her hubby Ozzy.

The 39-year-old singer, who is helping Sharon choose her final three contestants at the judges' house stage of this year's X Factor, has revealed that Mrs O once mistook him for her husband Ozzy Osbourne.

Robbie was staying in the same hotel as the Sharon when she phoned up his room, instead of Ozzy's.

Speaking to The Sun Robbie said: "Ozzy was staying in the next room and she phoned me by mistake and asked if I was naked.

"I picked it up, and she went, 'Are you naked, Daddy?'

"I went, 'Sharon - Ozzy's next door'. That was the very first time we 'met'."

Robbie also told the paper that he and Gary have sung on each other's new albums, which are due out over the next few months.

He said: "I've got a song on his record, and he's got one on mine.

"We did 'Requiem' for mine and then we did 'Wedding Bells' for Robbie's Swings Both Ways.

"It's a cheeky swing, big-band song."

He added: "We love making music but we're competitive too."