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Passengers have complained of queues and "total chaos" at Heathrow after the airport asked airlines to remove 30 flights from Thursday's schedule.
The UK's largest airport asked airlines to cut the flights because it was expecting more passenger numbers than it can currently cope with.
Some passengers did not know that their flights were cancelled until they arrived at the airport.
Heathrow said the cancellations were necessary for safety.
Travel writer and broadcaster Andy Mossack tweeted that there was "total chaos" and "zero customer service" at the airport on Thursday morning.
A Heathrow spokesman said: "We will work with airlines to get affected passengers rebooked onto other flights outside of the peak so that as many as possible can get away, and we apologise for the impact this has on travel plans.
"We are working hard to ensure everyone has a smooth journey through Heathrow this summer, and the most important thing is to make sure that all service providers at the airport have enough resources to meet demand."
A spokesman from British Airways, one of the airlines affected, said: "As a result of Heathrow's requirement for all airlines to reduce their schedules, we've made a small number of cancellations."
The airline said it was in contact with affected customers to "apologise, advise them of their consumer rights and offer them alternative options, including a refund or rebooking."