The passenger who was forcibly dragged from an overbooked United Airlines flight and left with blood dripping from his face may have been breaking the law by refusing to leave the aircraft, an expert in aviation law has said.

The man, reportedly a doctor called David Dao who needed to be at work in a hospital the next morning, was seen shouting while being violently removed from his seat and dragged down the aisle of the aircraft by security officers after he rejected calls from the crew to disembark.

He was filmed by horrified passengers who posted the video online, showing him with blood pouring from his mouth, his shirt pulled up around his chest and his glasses askew. Footage was later released showing the man saying: “I want to go home, I want to go home”.

The incident, which caused widespread outrage, occurred when the United Express Flight 3411 from Chicago O’Hare International Airport to Louisville, Kentucky, was overbooked and the airline required four passengers to leave the aircraft to make room for staff.

Passengers were offered first $400 and later $800 to give up their seats, but when too few people volunteered they became selected by the airline at random. Chicago security officers were called in when the man was chosen to leave but refused to.

Andrew Harakas, partner and aviation expert at Clyde & Co law firm, told The Independent that Mr Dao was effectively obliged to disembark under federal law if asked to do so by staff.

“Once you’re a passenger on board an aircraft you can’t interfere with the crew performing their duties or the aircraft being operated, that’s the basic rule,” he said.

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

“He was denied boarding, he should have got off the plane but he didn’t and the authorities were called. From a legal perspective, he was violating the law if he interfered with the crew members’ duties or the ability for the plane to be operated."

Mr Harakas stressed this was a highly unusual situation as the passengers were asked to disembark after having already boarded the aircraft, adding: “Until the facts all come down we can’t assess it either way.”

Footage of a man being dragged off a United Airlines flight sparked outrage on social media (Jayse D Anspach)

While incident is subject to US law, Mark Franklin, head of aviation at firm DLA Piper told The Independent the rules are practically the same in the UK.

“Once you are on board an aircraft it is a general requirement of conditions that you obey the orders of the pilot and cabin crew,” he said, while adding that the orders often relate to safety instructions such as requiring passengers to fasten their seatbelts.

“Once you disobey an order of the cabin crew you are potentially in trouble, however unreasonable it is,” he added.

The security officer who grabbed the passenger has been placed on leave, according to Chicago’s aviation department.

In an official statement, United Airlines’ chief executive Oscar Munoz said the incident had been an “upsetting event” and that his team was working with authorities and conducting its own review into what happened.