Heathrow Airport has ordered airlines to stop selling tickets for summer flights as it imposes a cap on passenger numbers.
No more than 100,000 daily departing passengers are permitted from Tuesday until September 11, the west London airport announced.
Airlines planned to operate flights with a daily capacity averaging 104,000 seats over that period, according to Heathrow.
That figure was much higher before carriers such as British Airways cancelled thousands of flights due to disruption across the aviation sector.
Around 131,000 passengers departed on Heathrow’s busiest day on record, August 4 2019.
The cap will lead to more cancellations, but passengers will not be entitled to compensation from airlines as the cause will be classified as outside their control.
Some carriers may choose to operate flights with empty seats.
Soaring air fares will increase further as the number of available seats is further cut.
In recent months, many passengers have faced long queues and not had their luggage put on their flights.
Airlines were able to take advantage of a Government scheme which meant they had until July 8 to cancel summer flights without losing their future rights to valuable take-off and landing slots.
Some airlines took “significant action” but “others have not”, according to Heathrow chief executive John Holland-Kaye.
In an open letter to passengers, he wrote: “Further action is needed now to ensure passengers have a safe and reliable journey.
“Our assessment is that the maximum number of daily departing passengers that airlines, airline ground handlers and the airport can collectively serve over the summer is no more than 100,000.
“The latest forecasts indicate that even despite the amnesty, daily departing seats over the summer will average 104,000 – giving a daily excess of 4,000 seats.
“On average only about 1,500 of these 4,000 daily seats have currently been sold to passengers, and so we are asking our airline partners to stop selling summer tickets to limit the impact on passengers.”
Mr Holland-Kaye admitted there are “some critical functions in the airport which are still significantly under-resourced”, such as ground handling.
A Virgin Atlantic spokeswoman said the company is “ready to deliver its full schedule this summer”.
She went on: “However, we support proactive measures being taken by Heathrow to reduce disruption, as long as action proposed does not disproportionately impact home carriers at the airport.
“Action should be based on thorough analysis showing the most effective measures to improve the situation and keep customers moving.
“We look forward to seeing Heathrow’s comprehensive plan for returning to normal operations as soon as possible.”
Guy Hobbs, acting editor of consumer magazine Which? Travel, said: “Thousands of people will now be worrying about whether their flight or holiday plans are about to fall apart.
“Heathrow must work with airlines to quickly provide clarity on which flights are being cut, and airlines need to be upfront with those passengers affected about their right to be rebooked at the earliest opportunity, including on services from other airlines.”
Tim Alderslade, chief executive of Airlines UK, the industry body representing UK-registered carriers, said: “UK airlines are working round the clock to deliver resilient summer schedules and Heathrow needs to play its full part in delivering for travellers.
“It is disappointing that Heathrow, which was forecasting lower passenger numbers and resources required during the recovery phase, has considered it necessary to take this action now to manage these shortcomings.
“The vast majority of flights from Heathrow are departing as planned this summer, and carriers will be in contact as early as possible with any customers affected by the cap.”
British Airways, the largest airline at Heathrow, was approached for a response.
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