UK air traffic experienced significant disruption on Monday following an air-traffic control system outage that caused massive delays and cancellations on one of the busiest days this travel season.

The NATS national air-traffic service said it’s “currently experiencing a technical issue and have applied traffic flow restrictions to maintain safety,” according to a statement. “Engineers are working to find and the fix the fault.”

While the UK air space hasn’t been closed outright, NATS said it has had to apply flow restrictions to maintain safety. Some international airlines were also affected, with Deutsche Lufthansa AG canceling flights from Frankfurt to Heathrow and Dutch carrier KLM circumnavigating the UK.

The glitch stands to disrupt travel and cause cancellations on one of the most active extended weekends, with the UK off on a national holiday on Monday and summer-vacation travelers returning home. Some 3,049 flights were due to depart UK airports on Monday, with about the same amount arriving, according to aviation analytics firm Cirium.

Swanwick Site

Eurocontrol, which regulates air traffic for the region, said the UK is experiencing “very high individual delays” because aircraft movements have been regulated at low rates. NATS operates its systems from the Swanwick operations room about 100 kilometers (60 miles) southwest of London that manages the airspace over England and Wales up to the Scottish border, as well as lower-altitude traffic to and from London’s airports.

“Currently there is no indication of when a solution for the failure will be available so no improvements for flights entering UK airspace are foreseen in the near future,” Eurocontrol said.

With delays and cancellations piling up, returning to normal service can take several days because aircraft will be put out of position. Ryanair Holdings Plc said it will be forced to delay or cancel “a number of flights” to and from the UK on Monday.

Data from Flightradar24 showed London Heathrow Airport had 34 canceled flights and 209 delays so far on Monday. Meanwhile, Gatwick Airport had 416 delayed flights and 11 cancelations. Some planes were being held on the ground at both hubs, according to Flightradar24’s maps.

Loganair, the Glasgow-based Scottish airline, wrote in a post on X, formerly known as Twitter, that a “network-wide failure” of air traffic control computer systems in the UK may cause delays to its flights. British Airways said it’s working closely with NATS to understand the impact of the technical glitch.

Swanwick opened almost six years late in 2002 and at least 30% over budget after repeated software glitches. Since then, the UK has experienced several major system outages, including one in 2014 that forced NATS to restrict the number of flying aircraft.

(Updates with details on airline delays from third paragraph.)