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BA faces paying out millions of pounds in compensation after the failure of its computer systems led to long delays and cancellations over the weekend.

Passengers are facing a third day of disruption as the airline deals with the impact of a worldwide IT crash.

There are EU regulations governing compensation for cancelled flights leaving from EU airports.

The amount of money reimbursed depends on the length of delay and whether it is a short, medium or long-haul flight.

A BA spokesman said: "We have been giving letters to customers telling them how to claim under EU compensation rules and we will fully honour our obligations."

Court rulings

Compensation could be more than £500, depending on the distance of the flight.

But compensation is not automatic - customers have to write a complaint letter to the airline.

Some airlines, and consumer body Which? provide standard compensation application letters for passengers to complete.

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EU compensation rules for delayed or cancelled flights

Passengers are entitled to assistance and compensation, if the disruption was within an airline's control

This applies to short-haul flights delayed by at least two hours,

And to medium-haul flights delayed by three hours, or long-haul delayed by four

For overnight delays, airlines must provide hotels, and transfers between airport and hotel

Airlines have to offer full refunds, paid within seven days, or re-bookings for a flight cancelled at short notice

In addition passengers can also claim compensation

Cancellation amounts are: 250 euros (£218) for short-haul, 440 euros (£384) for medium-haul and 600 euros (£523) for long-haul

Passengers who reach their destination more than three hours late can be compensated from 200 to 600 euros, depending on the length of flights and delay

The EU regulations do not apply if the disruption has been caused by factors outside the airline's control, such as a strike.

In 2014, two UK Supreme Court judgements stated airlines should have to pay out when a delay was caused by a technical fault, which appears to have happened in this instance with BA.

Aviation expert Julian Bray adds: "Also remember if you actually paid for your fare with a credit card, then the Consumer Credit Act comes into play, and you could well get money back that way."

Receipts

Meanwhile, BA says it is meeting its obligations in providing hotel accommodation and refreshments for customers whose journeys have been disrupted.

"We are refunding or rebooking customers who suffered cancellations on to new services as quickly as possible and have also introduced more flexible rebooking policies for anyone who was due to travel on Saturday, Sunday, today [Monday] and Tuesday who no longer wishes to fly to-from Heathrow or Gatwick," they add.

"Customers on flights that have been cancelled can claim a full refund or rebook to a future date for travel up until the end of November 2017.

"Customers should get in touch with us directly via our Manage My Booking tool on ba.com or our contact centres so that we can re-book or re-route them to their destination as quickly as possible."

They added: "Passengers whose journeys are disrupted have been urged to keep any food, transport or accommodation receipts and can make a claim in due course through our customer relations teams."

The firm says it has been booking passengers onto other airlines where necessary.

Customers displaced by flight cancellations can claim up to £200 a day for a room (based on two people sharing), £50 for transport between the hotel and airport, and £25 a day per adult for meals and refreshments.

Consumer expert Franky Brehany said travellers stranded in a "high-value city" like London may be able to claim more and should keep all receipts.