Justin Bieber has been accused of smoking marijuana before a flight.

The 19-year-old singer and his friends are alleged to have been using the substance as they made their way to the airport.

The pilot enlisted to fly the plane made the allegations, claiming he was astounded by what greeted him when the star's car door was opened.

"I almost had to put on an oxygen mask," the pilot told website TMZ.

Once in the air, Justin is said to have behaved well and refrained from smoking.

However, some of his friends are said to have lit up as they enjoyed their flight.

The pilot claims Justin wasn't always totally aware of what he was doing and got confused when he was signing a credit card bill.

"When he signed for his bill, he asked, 'Huh, what? Who am I signing this for?'" the pilot recalled.

This isn't the first time allegations about the pop star allegedly smoking marijuana have been made.

Photos circulated the internet at the beginning of the year which showed him puffing on a questionable-looking cigarette.

In February during a skit for Saturday Night Live, Justin took the moment to apologise for the incident.

In the sketch the singer was playing Pete, the make-believe president of Miley Cyrus' fan club, who hates Justin Bieber.

"I also heard he got busted for smoking weed and he's really sorry about it and people make mistakes and he's never gonna do it again," he said in character.

Justin had only previously addressed the photos by tweeting that he did not want to "disappoint" his fans.

In July, the singer's tour bus was stopped and seized for drugs during the North American leg of his Believe concert tour. It was alleged marijuana and drug paraphernalia were found on board.