Buff Birthday Boy Justin Bieber Loses His Shirt – And Nearly His Pants!

Maybe Justin Bieber should do a cover of the novelty song Pants On The Ground!

The surprisingly buff-looking Canadian teen heartthrob -- who turns 19 on Friday – celebrated his birthday a day early by going shirtless, with his pants hanging EXTREMELY low to expose his briefs, after his concert in Birmingham, England on Thursday night, and RadarOnline.com has the pics.

"Great show!!! Now gonna go celebrate 19!! Thanks," Biebs tweeted after the concert.

According to UK’s Mirror Online, Bieber -- who earlier this week raised eyebrows by wearing a black gas mask while shopping -- plans to celebrate his birthday with 20 pals at a circus-themed bash, featuring clowns and a ringmaster, at a London venue.