WASHINGTON (Reuters) -United Airlines Chief Executive Scott Kirby said on Tuesday that the business model of ultra-low-cost airlines is not viable.

The comments came after no-frills pioneer Spirit Airlines filed for bankruptcy protection last month for the second time in a year after a previous reorganization failed to put it on firmer financial footing.

"ULCC model was an interesting experiment, consumers voted and it failed," Kirby told the U.S. Chamber of Commerce's Global Aerospace Summit in Washington. "I don't think the ULCC model is viable."