Airlines are demanding answers from the air traffic control (ATC) provider NATS after a system outage grounded flights across the UK.
NATS, which has apologised, said it had taken 20 minutes to resolve the issue by switching to a back-up system.
It resulted in more than 150 flights being cancelled and thousands more being grounded on Wednesday.
Ten flights on Thursday morning to and from Heathrow Airport were cancelled – four departures and six arrivals, according to the airport’s website.
A spokesperson for Stansted Airport said all flights were operating as normal on Thursday morning following Wednesday’s disruption.
EasyJet said the error was “extremely disappointing”, while a Ryanair executive called for NATS chief executive Martin Rolfe to resign.
According to NATS, the issue was “radar-related” and it reduced flight traffic to ensure safety. It added there was no evidence the incident had been cyber-related.
The government said it was “working closely” with NATS to understand what had gone wrong.
Ryanair executive Neal McMahon said it was “outrageous that passengers were once again being hit with delays and disruption due to Martin Rolfe’s continued mismanagement” of NATS.
Referring to a previous incident two years ago, Mr McMahon said it was clear “no lessons” had been learnt and passengers continued to “suffer” as a result of Mr Rolfe’s “incompetence”.
In August 2023, more than 700,000 passengers were affected by the cancellation of more than 500 flights at the UK’s busiest airports due to a major outage.
EasyJet’s chief operating officer, David Morgan, said: “It’s extremely disappointing to see an ATC failure once again causing disruption to our customers at this busy and important time of year for travel.”
“We will want to understand from NATS what steps they are taking to ensure issues don’t continue,” he added.
Travel expert Simon Calder told BBC Breakfast there would be “thousands of people who are waking up where they did not expect to be this morning”.
He warned that disruption from Wednesday’s outage would continue in the coming days as airlines attempt to get stranded passengers, as well as planes and crew, to their intended destinations during the busy summer holiday schedule.
“The trouble is, at this time of year everything is stretched, all the airlines are running to maximum efficiency, very little slack in the system, very few spare seats to accommodate passengers whose flights were cancelled in the hundreds,” he said.
Sarah McPherson’s son was meant to compete with England’s under 15 boys international touch rugby team in Ireland, and she said she was “feeling pretty angry”.
“He is so very disappointed and at this stage we don’t know if there will be another flight,” she added.
Jonathan Carr from Stourbridge was trying to get to Norway for his brother’s wedding. He told the BBC it was “stressful” that his flight was cancelled but he hoped to arrive in Bodø late on Thursday.
“It’s really frustrating,” said Julia Lo Bue-Said, chief executive of Advantage Travel Partnership. “We need to ensure there’s sufficient resilience built into critical infrastructure that doesn’t impact businesses, that doesn’t impact travellers.”
According to flight data firm Cirium, 84 departures and 71 arrivals were cancelled across all UK airports. That equates to 3% of all departures and 2% of all arrivals.
In addition a number of flights had to be diverted to European airports, it said.
Cirium does not attribute causes for cancellations, so some may not have been due to the air traffic outage.