British Airways said that the disruption which caused chaos for thousands of Bank Holiday travellers had nothing to do with cutting costs.
Chief executive Alex Cruz said the computer failure was the result of a short power surge. The company had a back-up system but it had not worked properly.
He denied that the outage was due to technical staff being outsourced from the UK to India.
Mr Cruz offered apologies for the IT failure which grounded planes and caused hundreds of cancelled flights.
Two thirds of passengers will have reached their destination by the end of today, he said, and repeated claims on Saturday that there was no evidence of a cyber attack.
He said he would not resign despite widespread criticism of the company’s handling of the crisis which wiped £360 million off the value of parent company IAG.
All planes were grounded at Heathrow and Gatwick and the chaos spread to other airports.
BA said today it would run a full schedule at Gatwick and intended to operate a full long-haul schedule from Heathrow along with a high proportion of short-haul service.
However, 13 short-haul flights were cancelled on Monday morning. In a statement it said: “We continue to make good progress in rebuilding our operation, following Saturday’s major IT systems failure which severely affected our operations worldwide.
“At Heathrow, we have operated virtually all our scheduled long-haul flights, though the knock-on effects of Saturday’s disruption resulted in a reduced short-haul programme.
“We apologise again to customers for the frustration and inconvenience they are experiencing and thank them for their continued patience.”
The company is facing a huge compensation bill and PR exercise to restore its battered reputation with many passengers, critical of the lack of direct communication, saying they will never again use the airline.
Queues stretched around the terminal at Heathrow (above) as hundreds of passengers were forced to wait to check-in.
Travellers spent the night sleeping on yoga mats provided by BA. The disruption continued into Sunday, with queues building up as passengers tried to rebook flights. Conference rooms at the airport were opened to provide somewhere more comfortable for passengers to rest.
Some passengers who did get to travel had to do so without luggage.
The airline has said it will refund reasonable costs to passengers, and assist them with rebooking flights.
The GMB union has suggested the failure could have been avoided, had the airline not outsourced its IT work.
BA denied the claim, saying: “We would never compromise the integrity and security of our IT systems”.
All passengers affected by the failure – which coincides with the first weekend of the half-term holiday for many in the UK – will be offered the option of rescheduling or a refund.
The problems affected flights across Britain with many travellers accompanied by children heading for holiday breaks.
Parts of BA’s website have been unavailable and some travellers claimed they could not check in on the mobile app.
BA aircraft landing at Heathrow are unable to park up as outbound aircraft cannot vacate the gates, which has resulted in passengers being stuck on aircraft.
Delays were reported in Rome, Prague, Milan, Stockholm and Malaga due to the system failure.
BA is the latest airline to be hit by computer problems after Germany’s Lufthansa and Air France suffered a global system outage last month.