British Airways (BA) has said it hopes to run a "near normal schedule" at Gatwick and most services at Heathrow after a catastrophic IT failure caused all its flights from the busy airports to be cancelled.

Furious passengers told of chaotic scenes at airports around the world after the crash on Saturday, which BA said it thought was caused by a "power supply issue".

BA said on Saturday night that most long-haul flights into London on Sunday were expected to arrive as normal, but admitted some delays and disruption might continue for a second day over the bank holiday weekend.

The airline had announced an initial cancellation of flights until at least 6pm early on Saturday afternoon but airport staff had not been informed of the situation, and initially told passengers the information was false. BA later said all flights from the two airports had been cancelled for the remainder of the day.

Passengers at Heathrow were ordered to leave the terminal and told they would not be able to access their luggage for the rest of the day because the baggage system was down. One traveller claimed they had been told it could take a week for their bags to be returned, while another said she had been warned it could take three hours to exit the airport.

BA had been unable to update its website or make announcements at some airports because both rely on the system that has crashed. Staff resorted to writing on whiteboards in an attempt to keep passengers updated.

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 are extremely sorry for the inconvenience this is causing our customers and we are working to resolve the situation as quickly as possible.”

BA added: “We've found no evidence that it's a cyber attack.”

The company's chief executive, Alex Cruz, later added in a video statement: “Our IT teams are working tirelessly to fix the problems. We believe the root cause was a power supply issue.

“I am really sorry we don't have better news as yet, but I can assure you our teams are working as hard as they can to resolve these issues."

Customers have been left in the dark, with staff giving conflicting information or admitting they did not know what was going on. Customer service representatives are unable to tell passengers with flights booked later on Saturday whether it is likely they will be able to fly.

In pictures: British Airways disruptions Show all 17 1 /17 In pictures: British Airways disruptions In pictures: British Airways disruptions A passenger looks at a British Airway plane at John F. Kennedy (JFK) international airport in New York Getty Images In pictures: British Airways disruptions British Airways planes are seen at Heathrow Terminal 5 Reuters In pictures: British Airways disruptions Passengers stand at the British Airways check-in desk after the London's Gatwick and Heathrow airports suffered an IT systems failure, at the 'Leonardo da Vinci' airport in Fiumicino, near Rome, Italy EPA In pictures: British Airways disruptions Arrivals notice boards are displayed at Heathrow Terminal 5 Reuters In pictures: British Airways disruptions People wait with their luggage at the British Airways check in desks at Heathrow Terminal 5 Reuters In pictures: British Airways disruptions Thousands of passengers face a second day of travel disruption after a British Airways IT failure caused the airline to cancel most of its services Getty Images In pictures: British Airways disruptions A woman covered in a blanket sleeps in Heathrow Airport Terminal 5 Getty Images In pictures: British Airways disruptions People sleep next to their luggage at Heathrow Terminal 5 Reuters In pictures: British Airways disruptions People sleep at Heathrow Terminal 5 in London Reuters In pictures: British Airways disruptions A woman sleeps on a luggage trolley at Heathrow Terminal 5 Reuters In pictures: British Airways disruptions People queue to enter the terminal at Gatwick Airport Reuters In pictures: British Airways disruptions People wait with their luggage at Heathrow Airport Terminal 5 Getty In pictures: British Airways disruptions Thousands of passengers face a second day of travel disruption after a British Airways IT failure caused the airline to cancel most of its services Getty In pictures: British Airways disruptions People queue with their luggage outside Heathrow Airport Terminal 5 Getty Images In pictures: British Airways disruptions People queue for check-in at Heathrow Airport Terminal 5. Thousands of passengers face a second day of travel disruption after a British Airways IT failure caused the airline to cancel most of its services Getty Images In pictures: British Airways disruptions People sleep next to their luggage at Heathrow Terminal 5 Reuters In pictures: British Airways disruptions People wait with their luggage at Heathrow Terminal 5 Reuters

Emily Wilson, who was due to be flying from Heathrow Terminal 5 to Swedish capital Stockholm, told The Independent the airport was in chaos.

“The information kept changing – at first they said anyone who hadn’t checked in would be unable to fly today,” she said. “The staff were confused and were giving mixed messages to everyone.

“Then the news broke on Sky that said all flights before six would be cancelled and a lot of the staff were saying that wasn’t true, that wasn’t genuine information and they hadn’t been informed.

“There’s been a lot of tears from children and families. There are massive queues throughout T5 to try to get out of the airport.

“There have been so many mixed messages. There are people everywhere just sitting around because nobody knows what to do. The whole airport is in chaos.”

Airport chaos as BA cancels all flights from Heathrow and Gatwick

As the chaos unfolded, the GMB union, which represents many airline employees, claimed the failure could have been prevented.

Mick Rix, the union's national officer for aviation, said: "We can only feel genuinely sorry for the tens of thousands of passengers who are stranded at airports and face having their travel plans and holidays ruined.

"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."

Passengers were told it could take them three hours to leave the airport (Emily Wilson)

The company refuted the claim, saying: “We would never compromise the integrity and security of our IT systems.”

The failure occurred as tens of thousands of holidaymakers sought to leave the UK for the bank holiday weekend.

BA staff have told stranded passengers they cannot rebook them on later flights because the IT system remains down. A number of planes are stuck on the tarmac at Heathrow with passengers aboard because all the terminal gates are currently full.

A letter released by BA apologised for the disruption and said: “We recognise that this has impacted your travel plans.” The airline said it would be reimbursing "reasonable expenses" for meals and hotels, defined as £200 per day for two people sharing a room, and £50 for transport to the hotel. Food and refreshment costs would be covered up to £25 per day for adults and £12.50 for children.

Experts said the airline could face a huge compensation bill because the thousands of stranded passengers are entitled to compensation under EU law – unless BA can prove that the disruption was caused by factors outside its control.

Malcolm Ginsberg, editor in chief at Business Travel News, said: "There is no question: the EU denied-boarding regulations will have to apply.

“They have broken all the rules and they will have to deal with it – it’s going to be a very expensive situation for BA.”

At Heathrow, a BA announcement informed passengers: “We need you to leave the terminal and the way to leave the terminal is through gate number A12.

“There is a queue at the moment to get through to A12 because (we) can’t let you into the arrivals’ hall because that it too busy too, but you will have to leave through gate A12.

“You will have to try and rebook online or with another airline. Our systems are not open and we are not able to rebook any of you."

The announcement continued: “There will be no availability to pick up baggage today. Unfortunately our baggage system has failed too and we are not able to retrieve any bags.”

Passengers took to social media to seek information and express their frustration.

“Stranded in Vienna for three hours already and no indication as to whether we will even get back to Heathrow. Major lack of info,” one wrote.

“Can you at least tell people here stranded at Heathrow [what is happening] instead of [them] hearing about it on media”, another urged the airline.

Another passenger also requested information on what is going on, saying there had been “no info as to what is happening or how long the delay will be”.