LONDON — Britain's air traffic control system said it was hit by a technical issue Monday that threatened delays and cancellations of flights in and out of the country on a busy national holiday.

"We are currently experiencing a technical issue and have applied traffic flow restrictions to maintain safety," a spokesperson with Britain's National Air Traffic Service said just after 12:30 p.m. local time (7:30 a.m. ET).

The service said engineers were working to find and fix the fault. It was not clear how long it would take to resolve the issue.

In the meantime, a spokesperson said, air traffic personnel were having to enter flight plans manually.

"This morning’s technical issue is affecting our ability to automatically process flight plans," the spokesperson said. "Until our engineers have resolved this, flight plans are being input manually, which means we cannot process them at the same volume, hence we have applied traffic flow restrictions."

"Our technical experts are looking at all possible solutions to rectify this as quickly as possible," the spokesperson said.

London's biggest international hub, Heathrow Airport, said that "as a result of national airspace issues there is disruption to flights across the UK."

Stansted and Gatwick airports in London, as well as Manchester Airport in the north of England, said flights were affected.

The European air traffic authority, Eurocontrol, warned of “very high” delays because of a “flight data processing system failure” in the U.K.

Earlier, the Scottish airline Loganair said in a post on X, the social media platform formerly known as Twitter, that a "network-wide failure of UK air traffic control computer systems" had occurred early Monday.

The airline said it was hopeful it would be able to operate most intra-Scotland flights with a minimum of disruption, but it said north-south and international flights could be subject to delays.

Monday, a nationwide public holiday, is a busy travel day for U.K. residents.

A number of social media users said they were stuck on planes on the tarmac. NBC News was not immediately able to verify their accounts.

A traveler who told Reuters about being on a plane being held on the tarmac at Budapest, Hungary, said the pilot informed passengers that they were delayed because of a massive computer failure.

The traveler said passengers faced a possible eight- to 12-hour delay. NBC News was not immediately able to verify the report.