easyJet PLC warned Monday that its operating costs will be higher than expected in the second half of the year, reflecting additional leased aircraft, crew costs and airport charges.

The U.K. airline EZJ, -3.94% said that it will exceed the previously provided cost per available seat kilometer, or CASK, guidance. easyJet had said at its first-half results that its operating CASK for the second half ending Sept. 30 would be close to fiscal 2019 levels.

In addition, the company forecast its third-quarter capacity at around 87% of fiscal 2019 levels, and fourth quarter capacity at 90%.

easyJet said that the aviation industry is experiencing operational issues that are causing increased turnaround times and delayed departures, which have a knock-on effect resulting in flight cancellations. This has been worsened by a very tight labor market , it said.

“While in recent weeks the action we have taken to build in further resilience has seen us continue to operate up to 1,700 flights and carry up to a quarter of a million customers a day, the ongoing challenging operating environment has unfortunately continued to have an impact which has resulted in cancellations,” Chief Executive Johan Lundgren said.

The company said these capacity and cost impacts are a one-off this summer, and that it expects greater resilience in 2023.

Still, easyJet expects to operate 140,000 flights in the third quarter ending June 30, carrying around 22 million passengers, as demand for travel in April and May is seven times that of the same months in 2021.

The company said booking momentum continues in line with fiscal 2019 levels, and reported the award of an additional three aircraft-worth of slots at the Lisbon airport.

Write to Jaime Llinares Taboada at jaime.llinares@wsj.com; @JaimeLlinaresT