The president of the Transport Workers Union, the largest labor union at Southwest Airlines Co. (LUV) - Get Report has written a scathing letter that condemns "intolerable and cancerous" working conditions for the carrier's groundworkers and decries mistreatment including 2,700 disciplinary actions and 468 terminations since January 2015.

"Groundworkers are flagrantly mistreated and abused by management," wrote John Samuelsen, president of the New York-based union that represents 12,000 Southwest groundworkers as well as 15,000 flight attendants. The letter refers to the ground workers, members of TWU Local 555.

In 2017, Samuelsen said, "Southwest is writing up nearly three workers per day and firing one worker every other day."

"The outright hostility to the workforce has obliterated morale, which can only have a negative impact on the passenger experience," he wrote. "The TWU finds it hard to believe that Southwest finds this to be an ideal business model."

The letter, sent late Wednesday, was signed by Samuelsen, Local 555 President Greg Puriski and 13 other TWU leaders. They said they are available to meet with airline executives as soon as possible.

In early afternoon trading, Southwest shares were down 0.8%.

Russell McCrady, Southwest's vice president of labor relations, said the carrier is committed to "efforts to maintain strong, constructive relationships with our employees' representational groups including TWU 555.

"Discipline is a necessary part of business but any discipline we administer is far from 'arbitrary,'" McCrady said in a prepared statement. "We do not take for granted that Southwest continues to be named a best place to work and best employer by national publications and we are very proud that our employee culture is the foundation for these designations."

McCrady said Southwest will respond to Samuelsen's letter and welcomes the opportunity to meet.

Southwest employs about 54,000 workers including about 7,200 hired in both 2016 and 2015, said spokeswoman Beth Hardin. She said the number of terminations over three years is not atypical for the number of workers involved.

Local 555 signed a five-year contract in 2016. In an interview, Samuelsen said the letter is not related to contract negotiations but rather represents an effort "to fight Southwest on working conditions that are now entrenched on the property, on an antiquated labor relations model designed to drive production {that} drives morale down across Southwest properties."

TWU has about 200,000 members, including 42,000 in Local 100, which represents New York City bus and subway workers. Samuelsen headed Local 100 until he took over the TWU presidency in May.

"I'm a new president, taking note of a situation at Southwest where people are being fired and unfairly disciplined," he said. "I found that to be intolerable."

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This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.