A passenger who was violently dragged off an overbooked United Airlines flight was removed so staff could take his seat, a leaked email to airline employees has revealed.

Footage of the incident was shared widely on social media and has prompted outrage. The man – who said he was a doctor and needed to get home to treat patients the next day – was filmed with blood flowing down his chin afterwards, saying: “I want to go home, I want to go home."

United risks a backlash from passengers, with many people threatening to boycott the airline at the start of a busy holiday period.

United Airlines passenger is dragged off plane because airline overbooked

In a letter to employees, United Airlines' parent company chief executive Oscar Munoz revealed the company had been trying to make room for four employees of a partner airline, which resulted in four passengers being told to get off the flight from Chicago’s O’Hare International Airport to Louisville.

After the plane was fully boarded, the company’s “gate agents were approached by crew members that were told they needed to board the flight”, the message leaked to ABC said.

"We sought volunteers and then followed our involuntary denial of boarding process (including offering up to $1,000 in compensation) and when we approached one of these passengers to explain apologetically that he was being denied boarding, he raised his voice and refused to comply with crew member instructions," Mr Munoz said.

"He was approached a few more times after that in order to gain his compliance to come off the aircraft and each time he refused and became more and more disruptive and belligerent."

Mr Munoz said he was “upset” by the incident, but added that employees had “followed established procedures for dealing with situations like this”.

Man brutally dragged off United Airlines flight: "Please kill me"

The airline had at first asked for volunteers to give up their seats for the airline staff, offering $400 (£322) and then when no one came forward $800 (£645) per passenger. When no one agreed to leave the flight, United selected four passengers at random.

Three got off, but the fourth refused, saying he had patients he needed to treat.

Three city aviation department security officers got on the plane and, after two officers tried to reason with the man, a third pointed at him “basically saying, 'Sir, you have to get off the plane,”' said Tyler Bridges, a passenger whose wife, Audra D Bridges, posted a video on Facebook.

One of the security officers could be seen grabbing the screaming man from his window seat, across the armrest and dragging him down the aisle by his arms.

Other passengers on Flight 3411 are heard saying, “Please, my God,” ''What are you doing?“ ''This is wrong,” ''Look at what you did to him“ and ”Busted his lip.“

“We almost felt like we were being taken hostage,” said Tyler Bridges. “We were stuck there. You can't do anything as a traveller. You're relying on the airline.”

Chicago's aviation department said the security officer who grabbed the passenger had been placed on leave.

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“The incidence on United Flight 3411 was not in accordance with our standard operating procedure and the actions of the aviation security officer are obviously not condoned by the Department,' the department said in a statement.

After a three-hour delay, United Express Flight 3411 took off without the man aboard.

Airlines are allowed to sell more tickets than seats on the plane, and they routinely overbook flights because some people do not show up.

It is not unusual for airlines to offer travel vouchers to encourage people to give up their seats, and there are no rules for the process. When an airline demands that a passenger give up a seat, the airline is required to pay double the passenger's one-way fare, up to $675 (£544) provided the passenger is put on a flight that arrives within one to two hours of the original. The compensation rises to four times the ticket price, up to $1,350 (£1,088), for longer delays.

When they move passengers onto other flights, airlines are required to give those passengers a written description of their compensation rights.

Last year, United forced 3,765 people off oversold flights and another 62,895 United passengers volunteered to give up their seats, probably in exchange for travel vouchers. That is out of more than 86 million people who boarded a United flight in 2016, according to government figures. United ranks in the middle of US carriers when it comes to bumping passengers.