LONDON — Travellers on British Airways and its sister airlines in Spain faced a third day of delays and cancellations Monday, mainly on short-haul flights in Europe, after the company suffered a colossal IT failure over the weekend.

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

Data from flight tracker FlightAware.com showed BA’s sister airlines in Spain, Iberia and Air Nostrum, cancelled over 320 flights on Monday, a bank holiday in the U.K. that sees a high level of air travel.

BA itself cancelled another 27 flights and had 117 more delayed Monday.

The airline, which is part of the broader International Airlines Group, cancelled all flights from Heathrow and Gatwick on Saturday after the IT outage, which it blamed on a power-supply problem. The glitch threw the plans of tens of thousands of travellers into disarray.

Cruz told Sky News Monday the problem started at 9:30 Saturday morning 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.

BA 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 London’s Heathrow Airport, 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, BA’s 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.