The passenger who was brutally pulled off a United Airlines flight has been revealed as a doctor who once traded drugs for sex with a patient.

Dr David Dao, 69, was forcibly removed from a plane at Chicago's O'Hare Airport on Sunday when the flight was overbooked.

Video of the incident, which shows the man's limp body being dragged down the aisle by his hands and him bleeding from the mouth, has sparked a massive backlash against the airline for its heavy-handed approach and caused its share price to drop.

Dr Dao is now recovering in a Chicago hospital. When asked by Kentucky TV station WLKY what his injuries were, he said "everything" and added that he was not doing well.

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Now, Dr Dao's sordid criminal history has been published by multiple US media outlets.

The Vietnamese-born Kentucky man was convicted on multiple charges of obtaining drugs by fraud in 2004 and was placed on five years' probation, the Courier-Journal of Louisville reports.

The Kentucky medical licensing board investigated Dr Dao after his arrest and found he had become sexually interested in a male patient whom he physically examined, according to the New York Post.

The two began a relationship and would often meet for sex in motel rooms. Dr Dao would supply the patient with narcotics in exchange for sexual favours.

He eventually made the patient his office manager, but the man quit after Dr Dao made "inappropriate" remarks about him.

His medical licence was restored in 2015.

David Dao's glasses fall off his face as security guards drag him down the aisle of an overbooked United Airlines flight. Photo / Twitter
David Dao's glasses fall off his face as security guards drag him down the aisle of an overbooked United Airlines flight. Photo / Twitter

The doctor, who trained at a Vietnamese medical school before moving to the US, is a father of five. Four of his children are doctors, according to The Daily Mail.

His wife Teresa, who was also on the flight at the time of the incident, is a paediatrician.

Dr Dao is also a grandfather and a keen poker player.

There is no indication that United Airlines was aware of Dr Dao's past or that it affected their actions on Sunday.

Anger is growing against the airline, especially in China, where passengers are threatening to boycott after watching the video, which has been viewed more than 210 million times on social media site Weibo.

Many online commenters have accused United of having a racial bias against Asians.

The airline is struggling to contain the public relations disaster.

United chief executive officer Oscar Munoz has been slammed for calling the passenger "disruptive and belligerent".

He has since given a more conciliatory statement, apologising to customers.

"This is an upsetting event to all of us here at United," Mr Munoz said.

"I apologise for having to re-accommodate these customers.

"Our team is moving with a sense of urgency to work with the authorities and conduct our own detailed review of what happened.

"We are also reaching out to this passenger to talk directly to him and further address and resolve this situation."