WASHINGTON, March 10 (Reuters) – Several public advocacy groups on Thursday called on U.S. regulators to block a bid by budget carriers Frontier Group Holdings and Spirit Airlines Inc (SAVE.N) to create the fifth-largest U.S. airline, according to a letter seen by Reuters.

Public Citizen, Fight for the Future, the American Economic Liberties Project and other six groups said in a letter to the Transportation and Justice Departments that a merger between the ultra-low-cost carriers “would destroy competition in the only competitive market segment of the highly consolidated airline industry.”

The groups added: “Travelers will pay higher prices for fewer and lower quality options, workers will lose their jobs, and smaller firms will be muscled out of a once-competitive market segment.”