British Airways has resumed some flights from Britain's two biggest airports after a global computer system failure, but thousands of travellers face further chaos with about 40 flights cancelled from London's Heathrow airport.

About one quarter of Sunday's flights set to depart from London's key hub before 1pm (12:00 GMT) were cancelled as the airline battled to contain the "knock-on disruption" to schedules, with aircraft and crews out of position around the world.

British Airways said it was hoping to operate a near-normal schedule of flights from Gatwick, London's other major airport, and the majority of its Heathrow services.

More flights left Heathrow as the day wore on, but most were operating with delays. Travellers also faced hold-ups at Gatwick, but no flights were cancelled.

The airline advised passengers not to come to the two airports unless they had confirmed bookings, with disruption expected throughout the holiday weekend.

Monday is a public holiday and many children were starting a one-week school holiday.

Some passengers were curled up under blankets on the floor or sleeping slumped on luggage trolleys. Several passengers complained about a lack of information from BA representatives at the airport. Others said their luggage had been lost.

"Everyone is upset. There are people in tears," said Melanie Ware, who flew in from Los Angeles and was trying to get to Venice on her honeymoon.

"We rebooked for Venice for tonight, which they also have cancelled now," she told Sky News.

"So we have no way of getting out of Heathrow and they haven't compensated us for anything. We're stuck and this is the worst honeymoon ever.

"British Airways has ruined our honeymoon."

Possible cause

BA cancelled all its flights out of Heathrow and Gatwick on Saturday after the IT failure, which shut down all of the carrier's check-in and operational systems and affected call centres and its website.

"We believe the root cause was a power supply issue," said Alex Cruz, chairman and chief executive of BA, adding that IT teams were working "tirelessly" to fix the problems.

Full refunds will be given to customers who decide they no longer wish to travel, Cruz said.

While other airlines have been hit by computer problems, the scale and length of BA's troubles were unusual.

Delta Air Lines Inc cancelled hundreds of flights and delayed many others last August after an outage hit its computer systems.

Last month, Germany's Lufthansa and Air France suffered a global system outage which briefly prevented them from boarding passengers.