By Mica Rosenberg

NEWARK, N.J. (Reuters) - A close confidant of New Jersey Governor Chris Christie pleaded guilty on Thursday to his role in a bribery scheme involving United Airlines, in a case that emerged from the federal investigation into the "Bridgegate" scandal.

David Samson, 76, the former chairman of the Port Authority of New York and New Jersey, faces up to two years in prison under a plea agreement for using his position to pressure United into operating a flight between Newark, New Jersey, and Columbia, South Carolina, near where he owned a vacation home.

The plea deal does not require his cooperation with authorities.

The airline agreed to pay a $2.25 million penalty as part of a non-prosecution agreement with the government that also mandates improvements in the company's anti-bribery program.

The case grew out of the probe into the shutdown of lanes on the George Washington Bridge in September 2013 that caused massive traffic delays. Two former Christie allies have been charged with orchestrating that plot as political payback against a local mayor.

The airline scheme, which was unrelated to the bridge scandal, nevertheless served as a further embarrassment for Christie, who ran unsuccessfully for the Republican presidential nomination and has seen his approval ratings tank. Christie's office declined to comment.

The case also had an impact on United Continental Holdings Inc, whose chief executive Jeff Smisek and two other senior executives resigned in September 2015 after an internal probe into the matter.

"As we move forward, continuing to earn and keep the trust of our employees, customers, shareholders, and the communities we serve around the world remains critical to our success," Oscar Munoz, who took over as CEO after Smisek's resignation, said in a statement.

"CHAIRMAN'S FLIGHT"

The flights to Columbia from Newark Liberty International Airport, which is run by the Port Authority and where United sought improvements, were more than half-empty on average, according to U.S. government data. They ended after Samson resigned in 2014.

"This kind of case shakes public confidence in our institutions of government when people who are so accomplished, and who have occupied so many positions of public trust, misuse their authority to get something for themselves," Paul Fishman, the U.S. attorney in New Jersey, said in a statement.

In September 2011, Samson and Jamie Fox, a paid lobbyist for United who was Christie's former state transportation commissioner, had dinner with several United employees.

At the time, United was the largest carrier at Newark. The Port Authority's board was considering a proposal to build a hangar for wide-bodied planes, a project that stood to benefit United.

Samson and Fox urged United to restore the flight between Newark and Columbia, which had been canceled in 2009, prosecutors said. When airline representatives concluded the flight would lose money, Samson removed the hangar proposal from the board's agenda.

Following a series of email exchanges, the board took up the proposal again and approved it in December 2011. United then reinstated the flight, which Samson called the "Chairman's Flight," according to prosecutors.

Fox, 61, was charged on Thursday with conspiring to commit bribery.

Samson's attorney, Michael Chertoff, did not respond to a request for comment.

The Bridgegate scandal has led to charges against two former Christie allies: William Baroni, a former Port Authority official, and Bridget Kelly, the governor's former deputy chief of staff, are scheduled to face trial in September.

David Wildstein, also a former official at the Port Authority, has pleaded guilty and is cooperating.

(Additional reporting by Jeffery Dastin, Barbara Goldberg and Joseph Ax; Editing by Daniel Wallis and Tom Brown)