In pictures: What caused the UK air traffic control failure?

Air-traffic control services in the UK sought to restore normal operations after an hours-long system outage led to hundreds of delays and cancellations on one of the busiest days this travel season.

The NATS authority said it had "identified and remedied" the technical issue that knocked out the automated flight planning system earlier on Monday.

The system failure prevented planners from automatically processing flight plans and forced them instead to revert to manual input, meaning they couldn't manage the same volumes.

Heathrow and Gatwick airports both experienced steep delays and cancellations, with chaotic scenes playing out at London's two main hubs as the number of axed flights swelled.

The glitch coincided with one of the most active extended travel weekends, with the UK off on a national holiday on Monday and summer-vacation travelers returning back 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.

The failure occurred on a system that logs each aircraft's flight plan, which allows controllers monitoring flights to know where planes are headed and other important data, according to a statement by NATS.

The separate technology the company uses to track aircraft positions appeared to be functioning.

The issue had some similarities to a January 11 outage in the US that prompted a halt to all departures for a brief period and led to about 10,000 delays.

In that case, a Federal Aviation Administration computer that logged safety alerts covering such things as runway closures was shut down after contractors damaged an underlying database.

While the UK air space wasn't outright shut down, NATS said it has had to apply severe flow restrictions to maintain safety.

Some international airlines were also affected, with Deutsche Lufthansa canceling flights from Frankfurt to Heathrow and Dutch carrier KLM circumnavigating the UK.