Tens of thousands of British Airways passengers are experiencing delays due to what the airline described as “problems with the computer systems”.

Overnight UK time, travellers at airports worldwide – particularly in North America – turned up at check-in to discover long queues. The systems normally used globally for check-in failed, obliging staff to revert to manual issuing of boarding passes and luggage tags.

Staff at overseas airports were unable to issue onward boarding passes for connecting flights, requiring them to queue again when they arrived at Heathrow.

A spokesperson for BA said: “We are checking in customers at Heathrow and Gatwick Airport this morning, although it is taking longer than usual. We would encourage customers to check in online before they reach the airport. We are sorry for the delay to their journeys.”

The Independent has tracked delayed arrivals at Heathrow and Gatwick of more than an hour from about 20 airports, including Barbados, Orlando, Toronto, Washington and Hyderabad.

The most delayed UK airports in pictures Show all 10 1 /10 The most delayed UK airports in pictures The most delayed UK airports in pictures 1. Gatwick LONDON, ENGLAND - MARCH 22: A male police officer stands guard by an entrance to Gatwick North Terminal on March 22, 2016 in London, England. There has been an increased security presence at major London airports, including Gatwick, since the attacks in Brussels where a serious of explosions at the airport and a metro station have killed and injured people. (Photo by Chris Ratcliffe/Getty Images) Getty Images The most delayed UK airports in pictures 2. Luton File photo dated 10/11/2005 of London Luton Airport in Bedfordshire which is the most expensive in the country for pick-ups as motorists dropping off friends and relatives at airports this summer could be stung by "sky-high charges," the RAC has warned. PA The most delayed UK airports in pictures 3. Manchester MANCHESTER, ENGLAND - JUNE 28: Big Bird arrives at Manchester airport on his way to visit his friend Elmo at CBeebies The Furchester Hotel at Manchester Airport on June 28, 2016 in Manchester, England. (Photo by Shirlaine Forrest/Getty Images) Getty Images The most delayed UK airports in pictures 4. Glasgow File photo dated 25/05/11 of a plane landing at Glasgow Airport, which has won a Best in Europe award one week after being named Scottish Airport of the Year. PA The most delayed UK airports in pictures 5. Heathrow File photo dated 03/06/15 of passengers in the departure area of Heathrow Airport. Families jetting off on their summer holiday could be paying out nearly £62 a head extra on their break because of taxes on the must-buy items they want for their trip, according to new figures. PA The most delayed UK airports in pictures 6. Edinburgh The most delayed UK airports in pictures 7. Bristol BRISTOL, UNITED KINGDOM - JANUARY 7: A Continental Airlines aircaft bound for New York passes a grounded budget airline plane after numerous other airlines refused to land or take off after runway safety fears, at Bristol International Airport January 7, 2006 in Bristol, England. Thousands of passengers' journeys were disrupted this weekend as flights were cancelled, diverted or moved to neighbouring airports. (Photo by Matt Cardy/Getty Images) Getty Images The most delayed UK airports in pictures 8. Stansted BISHOP'S STORTFORD, ENGLAND - FEBRUARY 28: A general view of Stansted Airport on February 28, 2013 in Bishop's Stortford, England. The deal between Heathrow Airport Holdings, formerly known as BAA, and Manchester Airports Group to sell Stansted airport for ?1.5bn is to be finalised by today. The Competition Commission ruled that BAA must sell Stansted because of the lack of competition between London airports. (Photo by Bethany Clarke/Getty Images) Getty Images The most delayed UK airports in pictures 9. Birmingham A member of airport staff is seen among deserted check-in desks at the Birmingham International Airport in Birmingham, central England, on April 17, 2010. Britain has extended a ban on most flights in its airspace until 1:00 am (0000 GMT) Sunday, as the volcanic ash cloud from Iceland was "moving around and changing shape", air authorities said. Some 16,000 flights in European airspace on Saturday have been cancelled due to the cloud of volcanic ash from Iceland that is still lingering over the continent, Eurocontrol said. AFP PHOTO/BEN STANSALL (Photo credit should read BEN STANSALL/AFP/Getty Images) AFP/Getty Images The most delayed UK airports in pictures 10. Newcastle File photo dated 20/04/10 of a Jet2 plane at Newcastle Airport, as the budget airline announced that they will open its eighth UK base at Birmingham Airport next year. PA WIRE

With aircraft arriving late, delays built up for subsequent departures. The afternoon flight to Vancouver was delayed by three hours. Long-haul services to Las Vegas, Hong Kong, Tokyo, Lagos, Toronto, Houston and Bangalore were also substantially delayed.

Passengers have been told to turn up at the airport normally unless they are warned of a delay or cancellation, and British Airways has asked them to check in online.

Twelve short-haul flights to and from Heathrow have been cancelled during the day, though BA said these were due to separate issues unconnected with the check-in failure.

BA began to roll out a new check-in system, known as FLY, last October. Some front-line staff have expressed frustration about what they say are its inadequacies compared with the previous system.

Airlines are reliant on computer systems for speedy check-in, baggage handling and flight documentation. If these systems fail, the “manual fallback” is not always resilient.

Ten days ago, Monarch’s systems at Birmingham airport failed, leading to delays of four or five hours. A month ago, one of the world’s biggest airlines, Delta, had a system-wide failure which it said was caused by a power cut at its Atlanta hub.

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In the event of a delay of more than a few hours, the airline is obliged to provide meals and, if necessary, accommodation for passengers.