Justin Bieber reportedly wears gas masks in public to throw off paparazzi.

The 19-year-old singer first stepped out in the bizarre headgear in London several weeks ago and was again seen wearing the disguise in the city on Wednesday.

Apparently the fashion statement started out as a prank between Justin and his pal Julian Keyz. The music producer is reportedly one of several friends helping him to throw off photographers by wearing the mask.

"The gas mask Justin has been wearing around London is a joke between his friends, but people are starting to take it seriously," a source told Us Weekly magazine.

"All his friends take turns wearing it as a joke to throw off the paparazzi. It's not a serious thing, they're just having fun around London with it,” the insider added.

Justin has been in London for the week to perform a string of concerts at the city’s O2 Arena.

But the Canadian pop star’s trip to the UK has been riddled with controversy.

On Monday he arrived at his first in a series of four concerts two hours late, outraging parents of his young fans who had to be at school early this next morning.

On Thursday he collapsed backstage in the middle of his set after suffering from shortness of breath.

He was briefly hospitalised after finishing the show but got into a scuffle with paparazzi the next morning. Justin was reportedly heard yelling to a photographer during the altercation: "What the f**k you say? I'll f**king beat the f**k out of you."

He later took to Twitter in effort to explain some of the drama.

“Sometimes when people r shoving cameras in your face all day and yelling the worst thing possible at u… well I’m human. Rough week. (sic)” he tweeted.

The singer continued with his final show in London on Friday despite admitting he was still feeling “a little short of breath”.

He is believed to have turned to the famous father of his pal, Jaden Smith, in the wake of the controversy.

“love to big man Will Smith for the great talk yesterday," Justin tweeted on Saturday. "We know the truth and we must stand tall. Love u man! thanks!”

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