There was chaos for holidaymakers trying to fly out of London's Heathrow and Gatwick airports on Wednesday morning after British Airways' check-in systems crashed, causing long delays. Passengers described the situation on the ground as "total disorganisation", as pictures emerged showing huge queues at Heathrow in west London. At Gatwick, just to the south of the capital, computer problems also caused misery for BA customers, who reported delays of several hours. Many of those affected took to Twitter to vent their frustration at the airline, with some fearing they would miss their flights. Customer Sarah Ashdown wrote: “Total disorganisation @British_Airways at Heathrow. Stood in a queue for nearly 2 hours. Pretty sure I'll miss my flight thanks to you guys” Another described the scenes at Heathrow’s Terminal 5, where some of the longest queues formed, as “absolute carnage”. “@British_Airways T5 is absolute carnage with systems down!! Very unhappy customers left right and centre. #stillqueuing,” passenger Amanda Jayne Porter added. One traveller said the service offered by the British national carrier had been no better than some of its budget rivals. Paul Taylor tweeted: “@British_Airways premium airline? @easyJet @Ryanair must be having a good 'ole laugh. Where are your customer services people?” BA said later on Wednesday its check-in systems were functioning again. "Customers are being checked in as normal. We are sorry for the temporary check in problems which caused some delays for our customers first thing this morning," a spokeswoman told <em>The National</em>. “Our staff are working flat out to help customers get away on their holidays.” This is not the first time this year that BA customers have faced delays. In May, an estimated 75,000 passengers were grounded by a systems failure, thought to have been caused by a power supply problem. The latest failure comes at the busiest time of year for British airports as holidaymakers jet off during the school summer holidays. London, which is home to six commercial airports, is the world's busiest city airport system. Heathrow was named the sixth busiest airport in the world for passenger traffic in 2016, while British Airways is the largest airline in the United Kingdom based on fleet size. Elsewhere on Wednesday, Ryanair boss Michael O’Leary warned that disruptions to flights between Britain and the European Union would become more likely as the deadline for the end of the Brexit negotiations in March 2019 draws closer. Mr O’Leary made the comments after meeting British transport secretary Chris Grayling in London to discuss the need for an “open sky” deal between Britain and the EU, after the UK leaves the bloc. "It is now more likely there will be a disruption to flights," he said. "I think there will be disruption around September, October 2018." Airlines have warned that a deal needs to be decided by six months before the deadline for the Brexit talks, in order to allow for scheduled flights in the months immediately after Britain leaves the EU.