The look of shock does not even begin to describe Michael Buble's face when he heard a teenage fan sing with him on stage.

A mother recently interrupted Buble's concert to tell the singer her son, Sam Holyman, had just turned 15-years-old and was his biggest fan.

"I'm not doing anything, let's just talk," Buble said Paula Holyman interrupted his concert in Birmingham, England.

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"Are you telling me or asking me that he wants to sing? I know you're a mom, but I'm a singer. You know what? We've taken it this far, get him up here on stage," Buble told the woman.

Buble then instructed his orchestra to begin playing "Feeling Good".

Buble started crooning first, then it was Sam's turn.

"Holy s**tballs mom! Sam can sing!" Buble exclaimed.

Spontaneous applause erupted from the 14,000 crowd at the National Indoor Arena.

The video of the pair singing has gone viral - quickly picking up more than 3 million views on YouTube.