United Airlines chief Scott Kirby has forecast the demise of Spirit Airlines and signalled the decline of ultra-low-cost air travel, sparking backlash from budget carriers. He made the remarks during an industry event in California, reports London's Daily Mail.

Mr Kirby told the Airline Passenger Experience Association conference in Long Beach that Spirit's business model was unsustainable and unpopular with customers. He criticised the practice of offering low base fares and adding fees later, saying: "You can't have a business model predicated on 'screw the customer.'"

The executive repeated his criticism at another aviation event in Washington, prompting a sharp response from Spirit Airlines on social media. "Scott is finally right about something - it is all about customers," the airline posted on X.

Spirit, based in Florida, filed for Chapter 11 bankruptcy protection in August - its second such filing in less than a year. The airline first entered bankruptcy last November after years of financial losses, failed merger talks and mounting debt.

Spirit emerged from bankruptcy in March after creditors approved a restructuring plan that wiped out existing shares and transferred ownership to lenders including Citadel Advisors. It now carries US$2.4 billion in long-term debt and reported a negative free cash flow of US$1 billion at the end of the second quarter.

Last month, Spirit furloughed 270 pilots and demoted 140 more to cut costs. It also rejected a takeover offer from Frontier Airlines, saying the deal offered less value than its restructuring plan.

Spirit has not posted an annual profit since before the pandemic. A planned merger with Frontier in 2022 was derailed when JetBlue Airways made a higher bid, but that deal was blocked by the Department of Justice over anti-trust concerns.

Silver Airways, another regional carrier, also filed for Chapter 11 in December, raising concerns about travel disruptions to Florida and Caribbean destinations.