Airlines are demanding action after a second air traffic control failure in less than two weeks.
The latest disruption affected flights across Scotland, Northern Ireland, and northern England on Monday, with delays reported at airports including Edinburgh, Belfast, and Manchester.
National Air Traffic Services (Nats) confirmed the issue was separate from a previous software fault on September 8, which led to more than 2,000 flight cancellations and widespread passenger disruption.
A Nats spokesperson said: "The issue in our Prestwick centre is now fixed.
"We are working with airports and airlines to safely lift air traffic regulations as quickly as we can to minimise any further disruption.
"Passengers should still check the status of their flight with their airline.
"We apologise for the disruption."
The incident has renewed calls for accountability and improved system reliability from airlines.
EasyJet said: "This, once again, calls into question the resilience of Nats’ systems and demonstrates the need for firm actions to prevent these repeated failures.
"We will do all we can to minimise the impact of this for our customers and advise anyone due to fly today to check our flight tracker for the latest information."
Jet2 chief executive Steve Heapy said: "This morning’s disruption comes days after Nats published a report into its latest major failure and it is simply not good enough for this critical national infrastructure to keep letting customers down.
"Customers work hard to look forward to their holidays and airlines work hard to deliver those holidays.
"They should not be repeatedly exposed to delays and disruption because the UK’s air traffic control systems cannot be relied upon.
"There have been too many incidents, too many reports, and too many promises of lessons being learned.
"What customers and airlines need now is action, accountability and proper resilience, and we need it urgently.
"Make no mistake, we are counting the cost of this disruption and will be sending another bill to Nats."
British Airways said it was "disappointing" that some customers may experience disruption after "yet another technical fault with Nats".
Nats reported that airports south of Manchester had been broadly unaffected, but flights heading north and departing from Scottish airports had been experiencing some delays.
Transport Secretary Heidi Alexander said in a post on X that engineers had "fixed the problem and systems are resuming", but some delays may continue during the reset.
Nats is the UK’s primary provider of air traffic control services, managing around 2.5 million flights each year.
This is not the first time technical issues have caused major disruption.
In July, a radar-related fault at Nats led to more than 160 flight cancellations.
In August 2023, a technical glitch in flight plan processing grounded flights across the UK, affecting more than 700,000 passengers.
Airlines have previously called for the resignation of Nats chief executive Martin Rolfe following the repeated failures.
The latest incident has intensified industry calls for urgent reform and investment in the UK’s air traffic control infrastructure to prevent further disruption.
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