Flightradar24 data shows planes gathered at Heathrow and Gatwick airports on Monday, 28 August, as an air traffic control failure hit the UK.
Hundreds of thousands of airline passengers faced potential delays and cancellations on Bank Holiday Monday, one of the busiest days of the year.
Aircraft on the ground were being held and planes already in the air were likely to be diverted to other airports in Europe.
Eurocontrol, Europe's air traffic control agency, said Britain is "experiencing a flight data processing system failure" causing "very high" delays.
