Travelers faced a second day of cancellations after a technical problem at UK air traffic control provider NATS disrupted flights and left airlines struggling to restore their schedules.

According to flight-tracking service Flightradar24, 279 flights to and from UK airports had already been canceled on Wednesday, following around 1,300 cancellations on Tuesday. Departures from Heathrow accounted for most of Wednesday’s cancellations reported at that point.

Further disruption was expected at Heathrow, Gatwick, Manchester and Stansted as the effects of the previous day’s problems continued.

Travel journalist Simon Calder explained that canceled outbound services would also affect the following morning’s arrivals at Heathrow. Aircraft that had not departed could not operate their planned return flights, prolonging the disruption.

Airlines report widespread impact.

British Airways said it had canceled or diverted more than 100 flights on Tuesday, affecting tens of thousands of customers. Ryanair reported disruption to more than 65,000 passengers and said it had canceled over 50 scheduled services.

Some passengers described spending hours aboard aircraft waiting on runways. Reports included limited access to food, drinks and toilet facilities.

Airlines criticized Nats’ leadership over its handling of the incident. Responding to questions from Sky News, chief executive Martin Rolfe acknowledged his responsibility for the organization. He said his priorities were safety, resolving the fault and helping airlines, airports and passengers return to normal operations.

Check your flight before leaving home.

Luton Airport said early on Wednesday that it was open and operating, while warning that individual flights could still face disruption. It advised passengers to check their flight status directly before traveling to the airport.

For those with journeys affected by the outage, an operational airport did not necessarily mean their flight would run as scheduled. Continued cancellations showed that the disruption was still affecting travel plans the following day.