More than 600 flights were canceled on Tuesday due to a technical issue in Britain’s air traffic control system that caused disruptions at major airports across the U.K., the flight monitoring website Flightradar24 reported.

Britain’s National Air Traffic Services said a problem had been identified in its flight processing system and engineers were urgently working on the issue, which affected flights across the U.K.

In an update, it said it had “implemented a fix and our flight system is starting to recover.”

At least 625 flights to and from U.K. airports, including London Heathrow, were canceled, affecting services operated by multiple airlines, including British Airways, EasyJet, KLM and Lufthansa, according to Flightradar24.

Some passengers took to social media to complain about being stuck on the tarmac for hours, before their flights were eventually canceled. Many expressed frustration about a lack of communication from airlines.

The affected airports included Stansted and City in London, as well as those in Manchester, Birmingham, Gatwick and Scotland. They warned passengers to check the status of their flight with their airline.

Budget airline EasyJet said some of its flights were “unable to operate” and that the system failure caused “delays to flights across Europe.”