– Frontier Airlines said Friday that it will hire contractors to do the work of 1,300 employees — about a third of its total workforce — at the Denver airport and in Milwaukee.

Frontier said it will hire a unit of Swissport International Ltd. to perform all ground-operations work at the Denver airport, replacing 1,160 airline employees. Sitel, a privately held company based in Nashville, will be hired to replace 140 reservations workers in ­Milwaukee.

Frontier spokesman Todd Lehmacher said the Frontier employees would be given priority for interviews with the contractors. He said it was a "difficult but necessary decision" to shift the work to specialized contractors.

Before the cuts, Frontier had 3,800 employees, including pilots, mechanics, cabin crew and others.

Union officials said they were meeting with lawyers to determine their options. Steve Roberts, ­organizing director of the Transport Workers Union, called the outsourcing "bad business and bad for customers."

Denver-based Frontier has been remaking itself into a so-called ultra-low-cost airline similar to Spirit Airlines and Allegiant Air. That part of the industry has been growing rapidly.

Frontier's move follows an announcement this week by United Airlines that it is eliminating up to 2,000 jobs and hiring a contractor for baggage-handling and other work at 28 airports. After meeting with the International Association of Machinists, United said it would bring back jobs that had been outsourced at several other airports, although it didn't say how many jobs.