Justin Bieber losing his voice to puberty?

Teen superstar Justin Bieber is said to have taken on a vocal coach to stop him from losing his trademark voice during puberty.

The singer, who has complained that he now struggles to hit high notes in his breakout song 'Baby,' has drafted in a top trainer to help him.

A spokesperson for the star said: "Justin is physically developing at a slower rate than most guys so his voice is only breaking now. He's working with the best in the business, a specialist called Jan Smith, to make sure he gets much vocal rest as humanly possible for a working pop star."

Bieber's mentor Usher used the same voice coach to help him when his voice broke.

Concerns for his vocals were not enough to stop the teen from chatting to X Factor judge Cheryl Cole as he performed on the show last night. The 16 year old was seen making a 'call me' hand gesture towards her as he left the stage.

Later he tweeted, 'Cheryl u looked lovely as usual #sofine.'