Island Def Jam boss Steve Bartels has revealed to Billboard that Justin's voice is blossoming with age and his manager, Scooter Braun, has confirmed that "Vocally, his balls have dropped" on his new Christmas album 'Under the Mistletoe'.

Braun also revealed that a duet with Mariah Carey on her hit 'All I Want For Christmas Is You' posed a particular problem during recording, as she insisted that Bieber sang in her original key.

"That was the hardest song to do. A year ago that would have been no problem. Yes that was high, but I grinded it out and we got it down."

Bartels continued: "The album really shows his growth as a young man and his voice has just blossomed. It will pleasantly surprise many people. It's not your own typical holiday album."

'Mistletoe', Bieber's first single from the album, went straight in at No. 21 in The Official UK Top 40 Singles Chart last week (October 23).

Watch a new interview with the singer speaking about his Christmas album below, and fan footage of Justin Bieber singing 'Baby' with Taylor Swift at L.A.'s Staples Center.