Flights to and from the UK were experiencing disruptions after Britain's air traffic control systems were temporarily hit by a technical fault on Monday.
The National Air Traffic Services (NATS) said it had "identified and remedied" a technical issue which forced it to impose traffic flow restrictions earlier in the day.
"We are now working closely with airlines and airports to manage the flights affected as efficiently as possible. Our engineers will be carefully monitoring the system's performance as we return to normal operations," it said.
NATS said the issue impacted its ability to automatically process flight plans, "meaning that flight plans had to be processed manually which cannot be done at the same volume".
The issue hit on a busy travel day with Monday a public holiday in parts of the UK and triggered warnings of flight delays and cancellations from airlines and airports.
"We are seeing delays, and cancellations are likely," London's Gatwick airport said.
Heathrow, the UK's biggest and busiest airport, said it was "working closely with NATS and other airport partners to minimise the impact this has on passengers" and advised travellers check with their airline for the latest updates.
British Airways said that "like all airlines using UK airspace, our flights are being severely disrupted" and it had to make "significant changes" to its schedule.
BA advised that passengers on short-haul services today do not travel to the airport without checking the status of their flight "as it may no longer be operating".
Budget airline Ryanair, which mainly flies across Europe, meanwhile said that it will be "forced to delay/cancel a number of flights" to and from the UK on Monday.