British Airways could face huge compensation costs after thousands of passengers were stranded by a global IT crash.
Delayed travellers are able to claim compensation under EU law, unless the disruption has been caused by factors outside the airline's control.
Air travel experts say BA is likely to face a massive cost in lost revenue and payouts to customers whose flights were cancelled.
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."
He added: "The money doesn't really compensate passengers for the situation."
Airlines also have to provide food and drink if their passengers are delayed by more than two hours under the regulations.
Mr Ginsberg continued: "There is a statutory requirement on refreshments and it's up to BA to issue the vouchers and make sure that the waiting passengers are looked after."
Civil Aviation Authority guidance states that anyone who is more than three hours delayed arriving at their destination could be entitled to compensation.
"We would try to ensure BA are looking after their customers and advising them of their rights," a spokesman said.
A spokesman for consumer rights company Which? said: "If you're unlucky enough to experience a severe delay or cancellation, compensation is available and people should claim what they're rightly entitled to."
In a letter to customers at Gatwick Airport the airline said: "As part of our care obligations to our customers under EC Regulation 261/2004, we will be happy to consider reimbursing reasonable expenses for meals/refreshments and the provision of hotels, and transport to/from the hotel or other accommodation if an overnight stay is necessary based on the following guidelines:
:: Hotel accommodation (£200 per day per room - 2 people sharing).
:: Transport between the airport and your hotel (£50 for the round trip).
:: Reasonable meal and refreshment expenses (£25 per adult per day and £12.50 for children).
:: Two reasonable telephone calls per customer.
"Please accept our sincere apologies again for the disruption you have experienced today. We don't underestimate how inconvenient this is for you."