"The gas mask Justin has been wearing around London is a joke between his friends, but people are starting to take it seriously," the source tells Us Weekly. "Justin's close friend Julian Keyz (a young songwriter/producer also known as Julkeys) also wears it to pretend he's Justin and throw people off as they're the same height."
"All his friends take turns wearing it as a joke to throw off the paparazzi," the source continued. "It's not a serious thing, they're just having fun around London with it."
The 19-year-old singer was first spotted wearing the odd mask several weeks ago while shopping in Londontown, and he wore the disguise a second time on Wednesday, March 6, as he left famed restaurant Mr. Chow's.
Just one week earlier, the "Boyfriend" singer turned heads for a different reason -- wearing a bizarre ensemble of purple leopard-print harem pants, a black sweatshirt with illustrations of blue and yellow rolled-up bills, and a bright yellow cap with silver spikes.
Bieber later set the record straight with his fans, telling them that he had donned the unusual getup as part of a dare from his pal Ryan.
"Ryan said you won't go out in that, I said watch me," the prankster wrote on Instagram. "Haha, too colorful. Next time I'm gonna wear a black hat."
On Thursday, March 7, the singer was hospitalized after fainting mid-show during concert at London's O2 arena. After spending 20 minutes offstage recouping, he returned to the stage to finish off the show.
He later performed his final show in London on Friday, March 8, despite feeling "a little short of breath."
"So while I have seen some crazy stuff it is all smiles," he tweeted on Friday. "Im gonna get some rest and keep doing what i love. Entertain all of u. thanks."