Around 300,000 people were left stuck in airports in the UK and abroad after a lengthy air traffic control outage.

The National Air Traffic Services (NATS) IT system went down on Monday, a national holiday in the UK, which led to flight plans being slowly processed manually.

An analysis of flight data by the PA news agency found that at least 281 flights, including departures and arrivals, were canceled on Tuesday at the UK's six busiest airports.

The outage is estimated to have cost around £80 million ($101m).

"Very occasionally technical issues occur that are complex and take longer to resolve," NATS chief executive Martin Rolfe said. "In the event of such an issue, our systems are designed to isolate the problem and prioritize continued safe air traffic control."

NATS will provide a preliminary report to the Secretary of State for Transport next week, with the findings set to be made public.

The severity of the outage means that, even though the system is back online, it will take days for services to fully return to normal.