Frontier Airlines (F9, Denver International) has reportedly delayed its initial public offering (IPO) until at least September, reports Bloomberg, citing sources close to the matter. The Colorado-based airline lodged papers with the Securities and Exchange Commission (SEC) in March, but had not set an offering date.

Last week, Frontier announced a dramatic route expansion, adding twenty-one new cities to its network. "Customers will benefit not only from the broad new selection of nonstop routes, but our growing network will provide more than 1,000 new connecting route options," said Barry Biffle, Frontier's CEO, adding that passengers would be moving through Denver, its largest hub.

Responding to Frontier's expansion, President of United Airlines, Scott Kirby, said that it was a sign that Frontier's point-to-point model had failed.

"This is the first public validation that one of the ULCCs [ultra low-cost carriers] has thrown in the towel on the point-to-point business model," Kirby is reported as saying in Business Insider. "I can promise you that they are now competing on our turf as a network carrier in Denver. That is a battle I guarantee United will win."

Kirby also said that it was unusual for Frontier to pivot to a network business model in the middle of an IPO.

Frontier has not issued an official statement confirming the delay of the offering.