The chief executive of United Airlines has said he feels "shame and embarrassment" and vowed that a passenger will never be dragged screaming from one of its planes again.

Oscar Munoz was pilloried for his initial response to the incident in which he backed his staff after David Dao, 69, a doctor, was hauled off an overbooked flight by airport police in Chicago.

Amid a global backlash on social media and threats of boycotts by passengers Mr Munoz went on US television to issue an unqualified apology.

He said: "That is not who our family at United is. This will never happen again on a United flight. That's my promise. No one should ever be mistreated this way

"We are not going to put a law enforcement official onto a plane to take them off, to remove a booked, paid, seated passenger. We can't do that. I think my reaction to most issues is to get the facts and circumstances. But my initial words fell short of truly expressing the shame."

Mr Munoz said there had been a "system failure" and that was his fault. An online petition calling for him to step down as chief executive of America's third biggest airline had more than 45,000 signatures, but he refused to do so.