London, May 27: British Airways cancelled all its flights from London's Heathrow and Gatwick airports on Saturday as a global computer outage grounded the travel plans of tens of thousands of passengers, including several Indians, on a busy holiday weekend.

The airline blamed a "major IT system failure" but discounted the possibility of a cyber attack, like the one that brought down Britain's National Health Service on May 12. "We believe the root cause was a power-supply issue and we have no evidence of any cyber attack," BA chief executive Alex Cruz said.

One union leader rushed to point fingers at BA's decision to outsource IT jobs to India but he did not offer any evidence to substantiate the charge. He appeared to be trying to make a political point while chaos is unfolding at two of the busiest airports in the world.

The computers that handle aircraft in-flight were not affected and so safety was not compromised, analysts said.

BA initially said that all flights departing Heathrow and Gatwick were cancelled until 6pm but later extended this to the rest of the day.

The airline's flights on Sunday from Chennai, Bangalore and Hyderabad to London have been cancelled. Flights from Delhi and Mumbai are scheduled to operate on Sunday but the timing should be checked. BA does not operate any flight from Calcutta.

An appeal for assistance from the Indian government came from Anshu Upmanu, who was stuck at Zurich. Upmanu tweeted foreign minister Sushma Swaraj: "Madam, BA officials not helping. We don't even know whether our onward flight BA257 to Delhi is cancelled or not. Please help."

Dharmesh Karmokar tweeted BA: "Tried your app and online from a phone, laptop. Please help."

BA responded: "We're sorry, but ba.com is very busy at the moment.... it may be better to try again at another time."

Subrata Mukherjee in Stockholm, trying to return to Dallas, Texas, posted: "They served us a sandwich after four hours. They eventually asked us to leave the plane, but they didn't give any information on whether we'll be able to fly. I had to book a hotel for myself as I was advised to do so and ask for compensation later.... I will have to cancel a family event tomorrow."

Sona Vyas, "actress, writer and mummy to three girls. Also owns a dog. And a husband", wrote: "Disgusting customer service @ British Airways. Stuck in LHR (London Heathrow) with elderly parents and 3 young kids with no help. Cruise ship leaves tomorrow."

Other airlines did not appear to have been affected. At Heathrow, most of the problems were at BA's main hub in Terminal 5 from where Amanda Rajkumar tweeted: "BA systems completely kaput! Queues at the door!"

British Indian comedian Paul Chowdhry showed he had retained his sense of humour: "British Airways, have you tried turning it off and turning it back on again?"

Baggage mountains built up at Heathrow because suitcases kept coming in but not going out.

A spokeswoman for the airline said: "We have experienced a major IT system failure that is causing very severe disruption to our flight operations worldwide. We've found no evidence that it's a cyber attack.

"The terminals at Heathrow and Gatwick have become extremely congested.... so please do not come to the airports. We are extremely sorry for the inconvenience...."

Travellers were initially told to check the airline website and Twitter account for updates about the situation. But this was a catch-22 situation because these were down, too.

Since arriving aircraft could not get into the docking area because the space was taken by planes unable to leave, passengers had to sit on the tarmac for four or more hours, often without food or water.

On one flight, BA did not improve the mood of passengers by demanding money for food.

Nadia Whittley posted: "Stranded on the plane and they make us pay for food! Please retweet this indecency!"

A leader representing the GMB, the UK trade union, blamed BA's transfer of some of its IT work to India for some of the problems.

Mick Rix, national officer for aviation at the union, said: "This could have all been avoided. In 2016, BA made hundreds of dedicated and loyal IT staff redundant and outsourced the work to India. BA have made substantial profits for a number of years, and many viewed the company's actions as just plain greedy."

At that time, a BA spokesman had said a contract had been signed with an Indian company and "about 200 IT staff" would be affected. It was also pointed out that BA employs around 35,000 people in the UK and hires 1,000 people a year.

There is no evidence that outsourcing is in any way responsible for today's problems. BA has suffered through its IT system for some time - the latest systems crash is the sixth in a year.