Air traffic controllers are dealing with the busiest day in the UK's aviation history and warn that the skies are almost at full capacity.
The National Air Traffic Control Services (Nats) estimates that a record 8,800 planes will pass through UK airspace during the next 24 hours, as the school holidays begin in England and Wales.
A total of 770,000 flights will use UK airspace this summer, 40,000 more than last year.
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"The ageing design of UK airspace means we will soon reach the limits of what can be managed without delays rising significantly," said Nats director Jamie Hutchison.
The warning comes as the government launches a public consultation on the future of the aviation industry. Ministers have been facing calls to increase airport capacity, ITV News reports, but there is strong opposition from residents living nearby.
Local communities "are very obviously concerned" about the increase in air traffic, "but actually modernising [airspace] means we can keep aircraft higher for longer," Nats chief executive Martin Rolfe told the BBC.
"We can have them descend more steeply than they currently do because modern aircraft are more capable than the types of aircraft that were in service when this airspace was originally designed," he said.