LONDON — Travelers on British Airways faced a third day of delays and cancellations Monday, although most long-haul services were resumed, after a colossal information technology failure over the weekend caused chaos for thousands of passengers.

Chief executive Alex Cruz said the airline was running a “near-full operation” at London’s Gatwick Airport and planned to operate all scheduled long-haul services from Heathrow Airport. But he said there would still be delays, as well as some canceled short-haul flights.

Data from flight tracker FlightAware.com showed the airline canceled another 27 flights and had 135 more delayed Monday, a bank holiday in the U.K. that sees a high level of air travel.

Iberia and Air Nostrum, which like British Airways are part of the broader International Airlines Group and share some data, canceled more than 320 flights Monday.

On Saturday, British Airways canceled all flights from Heathrow and Gatwick after the IT outage, which it blamed on a power-supply problem. The glitch threw the plans of tens of thousands of travelers into disarray – by Sunday night, almost 600 British Airways flights had been grounded.

Cruz told Sky News on Monday that the problem started at 9:30a.m. Saturday when “there was indeed a power surge that had a catastrophic effect over some communications hardware which eventually affected all the messaging across our systems.” He said there was no evidence indicating the airline had come under cyberattack.

British Airways operates hundreds of flights from Heathrow and Gatwick on a typical day – and both are major hubs for worldwide travel.

Passengers, some of whom had spent the night at Heathrow, faced frustrating waits to learn if and when they could fly out.

Some endured hours-long lines to check in, reclaim lost luggage or rebook flights at Terminal 5, the British Airways hub at Heathrow. Many complained about a lack of information from the airline.

Cruz apologized in a video statement, saying: “I know this has been a horrible time for customers.”

The British union GMB linked the IT problems directly to the company’s decision to cut IT staff last year.

“This could have all been avoided. In 2016, BA made hundreds of dedicated and loyal IT staff redundant and outsourced the work to India,” said Mick Rix, national officer for aviation at the union.