DENVER -- The top two carriers at Denver International Airport have told state labor officials they are laying off a total of about 600 people.

Frontier Airlines notified the Colorado Department of Labor and Employment in a letter dated June 27 that it intended to lay off 456 employees beginning Sept. 1.

United Airlines sent two letters to the state dated June 17 saying 50 customer service representatives and 100 workers classified as "ramp servicemen" in Denver would be laid off beginning this week.

"Unfortunately, among other things, the unanticipated and unprecedented rise in the price of fuel has forced Frontier to take these steps in an attempt to remain competitive in this challenging environment," wrote Kevin Stocker, senior director of human resources at Frontier.

Frontier spokeswoman Lindsey Purves did not disclose whether the airline has informed other states of cuts, saying the airline wanted to notify employees first.

By law, employers with a certain number of workers in Colorado are required to notify the state of large job cuts.

Denver-based Frontier Airlines Holdings Inc. filed for bankruptcy protection in April and warned in June of job cuts without releasing a number. Trying to cut costs, the airline announced in June it would begin charging travelers $25 for checked luggage.

State labor department spokesman Bill Thoennes said Monday that flight attendants and pilots were among the 456 people Frontier was laying off, but he did not have immediate access to the job titles or exact number of which Frontier positions were being cut.

Frontier has about 6,000 employees.

Stocker wrote that except for pilots, Frontier expected workers would be laid off in waves, with some being laid off Sept. 1 and some being laid off Oct. 1.

"It's going to be difficult for DIA to maintain the volume of traffic that it's had." - Richard Wobbekind, CU Economist

Cuts Will Be Significant For DIA

Meanwhile United said its cuts were part of changes to "ensure its business processes are the most cost efficient."

United is the No. 1 carrier at Denver's airport while Frontier is No. 2.

University of Colorado economist Richard Wobbekind said the job cuts are a small number for Colorado and not a surprise given that the entire airline industry is struggling with high fuel costs. Several carriers have announced plans to shed flights.

Still the cuts are significant for Denver International Airport.

"DIA is a significant revenue generator," Wobbekind said. "With the airlines' announcement of cutbacks in the number of flights in September, it's going to be difficult for DIA to maintain the volume of traffic that it's had."

Frontier Airlines Holdings Inc. said Monday that preliminary results for June showed mainline traffic fell 1.1 percent to 925.9 million revenue passenger miles, compared with June 2007. Capacity fell 1.3 percent to 1.06 billion available seat miles. Load factor, which measures occupancy, rose to 87.2 percent, up 0.2 points from June 2007.

United said last week that its June passenger traffic fell 3.6 percent from June 2007 to 10.34 billion revenue passenger miles. Capacity fell 0.6 percent to 11.97 billion available seat miles while load factor fell 2.6 percentage points to 86.5 percent.