The head of United Airlines has issued a grovelling apology to a doctor dragged off one of its flights as the US carrier battles to address a global tsunami of reputational damage and a potential legal move from the passenger. United chief executive Oscar Munoz issued the belated apology to the 69-year-old doctor, identified as David Dao,  who was forcibly dragged from a United Express plane in Chicago on Sunday so a  United crew member could take his seat. But that was not before videos of the incident swept around the world, prompting calls for boycotts and sparking an angry backlash in the fast-growing and strategically important Chinese market. Memes have already started spreading through social media lampooning the airline’s brutal treatment of its passenger Even White House spokesman Sean Spicer found the video of Dao’s ordeal  “troubling”. Read: The viral Video hits Delta did not want. Munoz had already issued one apology that had been criticised for its “tone-deaf’  wording.   He also sent an email to staff saying that while he deeply regretted the situation, he emphatically stood behind them and commending them for “continuing to go above and beyond to ensure we fly  right’’. “The truly horrific event that occurred on this flight has elicited many responses from all of us: outrage, anger, disappointment,’’ Munoz said in his second effort, released on Tuesday.  “I share all of those sentiments, and one above all: my deepest apologies for what happened. “Like you, I continue to be disturbed by what happened on this flight and I deeply apologize to the customer forcibly removed and to all the customers aboard. No one should ever be mistreated this way. “I want you to know that we take full responsibility and we will work to make it right. “It’s never too late to do the right thing. I have committed to our customers and our employees that we are going to fix what’s broken so this never happens again.” Dao was dragged from the plane after United failed to find volunteers willing to be bumped from the flight so that four its crew could take their seats. This was despite offering vouchers worth up to $US1000. It then selected four passengers to be removed from the plane under its involuntary denied boarding policy. Three went quietly but Dao refused, saying he had to get home to treat patients. Security was called and Dao was dragged bleeding from the plane as other passengers protested. A United report included in the email from Munoz to staff said the passenger raised his voice and refused to comply with crew member instructions. “He was approached a few more times after that in order gain his compliance to come off the aircraft and each time he refused and became more disruptive and belligerent,’’ it said, adding that staff had no choice but to call security agents Munoz pledged to conduct a  thorough review of crew movement, the airline’s policies for offering incentives for volunteers when flights are overbooked and how the airline handles oversold situations. It would also look at how it partners with airport authorities and local law enforcement. “We’ll communicate the results of our review by April 30,’’ he said. Controversy has also flared over US media reports of Dao’s criminal past as he recovered in a Chicago hospital. A father of five and a grandfather who went to medical school in Vietnam, a Kentucky newspaper revealed his US medical licence was suspended for illegally prescribing painkillers amid allegations he did so with one patient in return for sex. The Los Angeles Times reported medical documents showed that Dao had a history of mental health problems including depression, anxiety and post-traumatic stress disorder.  He had also been ordered to seek evaluation for “anger management” issues. He fought to have his licence re-instated and underwent an “educational intervention plan’’   involving retraining and monitoring before it was restored in 2015. Dao has now engaged Chicago lawyers Stephen Golan and Thomas Demetrio to act on the family’s  behalf. The lawyers said in a statement: “Dr. David Dao, the United Airlines passenger who was removed from an airplane seat on United Airlines Sunday, is undergoing treatment in a Chicago hospital for his injuries. “The family of Dr. Dao wants the world to know that they are very appreciative of the outpouring of prayers, concern and support they have received.  Currently, they are focused only on Dr. Dao’s medical care and treatment.”