Downing Street has said the head of the UK's main air traffic control provider has the confidence of government despite widespread calls for his resignation from airlines.
UK Transport Secretary Heidi Alexander will question NATS CEO Martin Rolfe as disruption continues with hundreds of flights cancelled following a technical issue.
Flight monitoring website Flightradar24 said 1,300 flights to and from UK airports were cancelled yesterday after a system issue hit the National Air Traffic Service, which provides air traffic control services to 15 UK airports.
Tens of thousands of passengers were caught up in the chaos, which has continued into today with further cancellations and delays expected as airlines deal with the knock-on impacts at airports including Heathrow, Gatwick, Manchester and Stansted.
More than 330,000 passengers were impacted by the flight disruption, according to industry body Airlines UK.
In a letter to the Transport Secretary today, the association's chief executive Tim Alderslade said more than 155,000 passengers had had flights cancelled while flights had been delayed for more than 180,000 passengers.
According to FlightRadar, 246 flights had been cancelled at Heathrow Airport by 6am, while Gatwick passengers faced 33 cancellations.
NATS has said the problem has since been resolved.
Fight schedules are returning to normal in Ireland following disruption to dozens of incoming and outgoing flights at Cork, Dublin and Shannon airports yesterday.
Around 90 flights to and from Dublin Airport were cancelled yesterday due to the issue.
Ryanair's Chief Operating Officer Neal McMahon called on Mr Rolfe to resign.
He said: "How many more failures do passengers have to suffer before overpaid failure NATS CEO Martin Rolfe accepts responsibility for his repeated mismanagement and resigns?
"Over three years after NATS' catastrophic 2023 system meltdown, it's clear that nothing has changed, with UK passengers once again suffering avoidable ATC (air traffic control) delays of up to eight hours today due a critical NATS ATC system failing."
Ryanair cancelled 260 flights, leading to an immediate loss of £2.5 million to £3 million (€1.9-3.4 million), its CEO Michael O'Leary told Reuters.
"We're looking at probably (losses of) five, six million or something," Mr O'Leary said, adding that they expected 30 more flights to be cancelled today as crews are in the wrong places and working the wrong hours.
Ms Alexander will speak with Mr Rolfe before she addresses MPs in the Commons for the second reading of the Civil Aviation Bill.
Mr Rolfe apologised for the issue, adding that he "always accept(s) full responsibility" when asked if he was considering his position after the incident.
Speaking to the BBC, he said: "Inevitably, things do go wrong in complex systems.
"We never want them to. We always hope they won’t, but we absolutely plan for them to and make sure we handle them properly."
Mr Rolfe added: "I would say the UK has one of the best reputations in the world for its air traffic control service, both from a safety perspective and from a resilience perspective.
"NATS was actually a partial privatisation 25 years ago, and since that point, it’s had access to funding.
"We’ve invested way more than a billion in technology over that time, our service has consistently improved. Our safety is consistently improved from a very good base.
"But all of those things don’t detract from the fact that when these things happen, it is disappointing and upsetting, for which I apologise."
On the cause of the problem, Mr Rolfe said a "thorough" investigation would take place, but ruled out a cyber attack.
He added: "My focus and that of the team has been on both getting our system operating back up normally, which it is now, but there’s obviously a huge tail to the disruption and we’ll be working on that through the course of today.
"And then our attention will turn to exactly what’s happened in this particular incident.
"What I will say is, I don’t believe we’ve ever seen an incident happen more than once, so this will be something different that we’ve never seen in 50 years of operation."
Dublin Airport said it expected close to a full schedule of flights to operate today but that the potential for some delays remained because of a knock-on effect from the disruption yesterday. Intending passengers are advised to check their airline's website for updates.
Speaking on RTÉ's Morning Ireland, Graeme McQueen, media relations manager at daa, described yesterday as challenging and warned that there is the potential for knock-on impacts elsewhere on the network in the UK and Europe.
He advised passengers to "stay close to their airlines' apps and websites" and check their emails.
"We are expecting pretty much a full schedule today.
Listen: Graeme McQueen of daa gives advice to passengers travelling
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"The airlines here, Ryanair, Emerald, and Air Lingus worked very hard to get all their planes back in position for this morning's busy first wave, which is taking off as normal at the moment. But there is that potential for disruption on the network," he said.
He said that many frustrated passengers stayed at the airport overnight, with teams on hand providing blankets and bottles of water, and that a sizable number of people were still at the airport.
Aer Lingus cancelled one flight to Malaga but passengers have been reaccommodated on a bigger plane later this afternoon, he added.
"We're still in that sort of end of summer period where it is busy do a lot of people were hanging around, but teams are working very, very hard today again to reaccommodate them," he said.
Cork Airport said that all flight operations have returned to normal this morning after nine flights were cancelled yesterday.
A spokesperson confirmed that the cancelled flights were five inbound flights and four outbound flights.
"Normal operations are expected at Cork Airport throughout the day. Passengers intending to travel today are advised to refer to their airline's website or mobile app for the latest and most up to date flight information," they said.
A spokesperson for Shannon Airport said that the vast majority of scheduled services were operating as planned.
"Yesterday's disruption impacted a number of flights to and from Shannon Airport, including Ryanair flight FR3258 from Shannon to Birmingham, Aer Lingus flights EI386 and EI388 from Shannon to London Heathrow, and Ryanair services FR2303 and FR2304 to and from Faro.
"While normal operations have resumed across the network today, Aer Lingus services EI380 and EI381 to and from London Heathrow were cancelled this morning as schedules recovered," they said.
Travel journalist Simon Calder said the knock-on effects today are "going to be horrible".
He told Sky News: "There are going to be dozens of cancellations in the morning of flights coming into Heathrow because, basically, the flights didn't go out tonight."
British Airways (BA) said it was forced to cancel or divert more than 100 flights yesterday which had impacted "tens of thousands of customers".
Read more: Ryanair, Wizz Air call for NATS overhaul after UK flight disruption
Among the thousands facing disruption yesterday was Josie Donoghue, who had to get off her flight to Ireland after spending three hours on the runway at Gatwick.
Ms Donoghue said getting through the airport "was like trying to get out of Alcatraz".
"We were trapped in there. We were told we couldn't get out until the flights were cancelled, and then eventually they allowed us to get out," she said.
"It's just an absolute nightmare down there, it's the only word I can use to describe it," she added.
Additional reporting by Reuters
