A "network-wide" computer malfunction forced the suspension of takeoffs and inbound aircraft, causing cancellations and delays of up to 12 hours for travelers leaving the UK and those coming back from vacations.

A few flights were able to take off, but air traffic was severely constrained while engineers worked to identify and fix the issue.

Prior to Nats, the national aviation controllers, announcing at 3.15pm that it had "identified and remedied" the problem that had arisen over four hours earlier, some 500 flights were canceled and others were delayed for hours.

Airport travelers in the UK and other parts of Europe complained of having their travel plans derailed, check-in counters closing, and airlines unable to guarantee whether their flights would depart.

Several days of interruption are predicted due to the abnormally protracted outage, which will also result in delays from crew and planes that were left in the wrong place.

Heathrow on Monday evening asked travelers to arrive at the airport only if flights were confirmed as operational. British Airways said passengers due to travel on Monday or Tuesday could rearrange their flights free of charge.

Monday afternoon saw almost all of easyJet's foreign departures at Gatwick, where roughly 150 flights were canceled. Although the airline was unable to confirm which flights will fly on Tuesday, it is expected that some ongoing effects would be felt on its itineraries.

In a statement, Gatwick stated that they would "operate on a normal schedule" on Tuesday but encouraged travellers to "check the status of their flight with the airline before travelling to the airport."

Flight tracking websites indicate that delays started at 11.30 am. At around 12.10 p.m., Nats acknowledged the issue, stating that it was "currently experiencing a technical issue" and that "traffic flow restrictions have been applied to maintain safety."

Following its announcement that the first problem had been resolved, Nats stated: "We are currently working closely with airlines and airports to manage the affected flights as effectively as possible. As we resume regular operations, our engineers will closely monitor the system's performance.

"Our top concern is always ensuring the safety of every flight in the UK, and we truly apologize for the inconvenience this is creating. To learn more about how this might affect your flight, please contact your airline.

By 2.30pm, slightly under 10% of all services—232 flights from the UK to other countries and 271 to other countries—had been canceled, according to data from the analytics company Cirium.

Schedules will "remain significantly disrupted for the rest of the day," according to a Heathrow official. "We ask passengers to only go to the airport if their flight is confirmed to still be operating," they continued. Teams at Heathrow are making every effort to lessen the ripple effects and help anyone whose travel plans have been impacted.

"Like all airlines using UK airspace, our flights have been severely disrupted," a British Airways representative said. Although Nats has now fixed the problem, there have been major and unforeseen delays and cancellations. We apologize for the extreme inconvenience caused and are making every effort to reroute clients whose flights have been impacted.

Travel agency Tui informed its clients that "significant delays to some of our flights" were to be expected.

Many of the flights from Ireland that were impacted were scheduled to go via UK airspace.

Along with vacationers, British athletes and others traveling back from the World Athletics Championships in Hungary were among the passengers affected by delays.

The BBC host Gabby Logan announced on X that her plane was stalled on the runway at the airport in Budapest and continued, "After over three weeks away from home, I am hours from hugging my family. And I recently learned that UK airspace is closed. We might stay here for a full day. We wait while seated in the aircraft.

Since the outage—possibly the most serious since the company's control center in Swanwick, Hampshire, debuted in 2002—occurred on one of the most important travel dates in the calendar, Nats engineers will have been working against the clock to minimize the consequences.

In spite of the fact that the airspace was only restricted for a short period of time, aircraft were impacted by a computer error at Nats in 2014 till the next day.

Before Mark Harper, the transport secretary, tweeted on X on Monday afternoon that ministers were "doing all we can," Labour and the Liberal Democrats questioned the government's apparent lack of action.

Although there are restrictions on traffic movement, UK airspace is still open, according to Harper. Aviation Minister Charlotte Vere and I are doing everything we can to support Nats as they work quickly to remedy this.

Prior to that, Louise Haigh, the shadow transport secretary, wrote on Facebook that the situation was "extremely concerning for passengers traveling in and out of the UK on one of the busiest days of the year," and that she was "surprised" that a ministerial statement hadn't been made.

The Lib Dems requested that Rishi Sunak hold a meeting of the Cobra. Wera Hobhouse, the party's spokesperson on transportation, said: "Millions of vacationers could experience significant disruption in the coming days due to this fault, and we can't risk this government being absent from the scene once more."