Shares of United Continental Holdings plunged more than 12 percent on Thursday, following a tense earnings call, in which CEO Oscar Munoz said the airline dug itself "in a hole" in regard to competition.
It was the biggest one-day percentage decline for the company's stock since October 2009, a year before United and Continental closed their merger.
The airline beat analysts' earnings expectations for the quarter ended in September, but executives said United has struggled to fend off competition from low-cost carriers and faced higher fuel and other costs.
Cost pressure could eat into the company's bottom line next year, said Joseph DeNardi, an analyst and managing director at Stifel, adding that his profit "estimates for next year will probably come down."
One of United's strategies to fight off low-cost competitors has been to increase capacity, or the number of seats the airline flies, but at the same time, the company's fuel and other costs are rising.
The company's shares were already trading lower but fell further after executives declined to provide more insight into 2018.
"I know this won't help matters much, but I know everybody's getting scared about the fact that we're not going to get into these numbers because of some ominous reason," Munoz said. "I cannot fully express to you how much in the middle of things that we are."
United has struggled with other solutions to counter pressure from low-cost competitors. Earlier this year, it rolled out its own no-frills product, basic economy, to compete, but last month its chief financial officer said the carrier lost market share after launching the new class of service, which prohibits passengers from using overhead bins or selecting their seats.
United told investors on Thursday that passenger revenue for each seat it flies per mile, a key industry metric, would fall between 1 and 3 percent in the last three months of the year.
The airline is also expanding its service in the fourth quarter by about 3.5 percent, it said in a presentation. That increased capacity is part of a strategy to win over more consumers as low-cost carriers offer rock-bottom fares.
For example, Scott Kirby, United's president, said that some low-cost airline fares have dropped to about $10 and that about 17 percent of United's revenue comes from markets where ultra-low-cost carriers, such as Spirit Airlines, operate, Kirby added.
United's stock is down nearly 18 percent so far this year, compared with a more than 6 percent gain in the shares of rival Delta Air Lines and a more than 10 percent rise in American Airlines' stock. However, all large U.S. airline stocks were lower Thursday afternoon.
Correction: An earlier version misstated the year-to-date percentage changes for shares of Delta and American Airlines.