Justin Bieber leaves London's BLC (The British Luxury Club) after celebrating his 19th birthday with friends. (Craig Harris/WENN.COM)

Reality TV star Jack Osbourne is worried Justin Bieber will "unravel" as his bad behaviour continues to hit headlines around the world.

The Baby hitmaker made news for all the wrong reasons during his recent U.K. visit with antics that included a birthday bust-up with bouncers at a London nightclub and a foul-mouthed altercation with a photographer on Friday.

He also kept fans waiting for the first gig of his residency at the city's O2 Arena on Monday, endured a trip to hospital after suffering breathing difficulties during another show there, and has since cancelled an upcoming concert in Portugal - and Osbourne is worried life in the spotlight has become too much to handle for the 19-year-old popstar.

In a series of posts on his Twitter.com page, he writes, "Ok it's (is) it me are (or) is Justin Bieber literally about to unravel in front of our very eyes? Hope it doesn't get too crazy for the young lad. PS someone clearly didn't tell him to not f**k with the British press."

The pop superstar was rumoured to have axed his Tuesday night (12Mar13) show in Lisbon, Portugal because of slow ticket sales, but his manager Scooter Braun has since dismissed the gossip, telling GossipCop.com the concert was sold out and only cancelled because of "issues" with dismantling the stage set after the gig.