Heathrow Airport will drastically cut the number of passengers who can go through the travel terminal each day.
Airport bosses have introduced a cap of 100,000 daily departing passengers until September 11, as the aviation sector struggles to cope with demand for travel.
The airport said it has ordered airlines to "stop selling summer tickets to limit the impact on passengers".
Some flights will be moved to another day, another airport or simply be cancelled.
Heathrow bosses suggested some airlines were to blame for not reducing the number of flights they were planning this summer.
Heathrow chief executive John Holland-Kaye said: "Over the past few weeks, as departing passenger numbers have regularly exceeded 100,000 a day, we have started to see periods when service drops to a level that is not acceptable: long queue times, delays for passengers requiring assistance, bags not travelling with passengers or arriving late, low punctuality and last-minute cancellations.
"This is due to a combination of reduced arrivals punctuality (as a result of delays at other airports and in European airspace) and increased passenger numbers starting to exceed the combined capacity of airlines, airline ground handlers and the airport.
"Our colleagues are going above and beyond to get as many passengers away as possible, but we cannot put them at risk for their own safety and wellbeing."
It is not currently clear how Heathrow will limit the numbers, and whether whole flights or individual passengers will be impacted.
The airport predicts that 1,500 of the 4,000 seats that need to be cut have been sold so far, and that a ticket selling ban would stop the additional 2,500 being hoovered up.
Heathrow has been struggling to meet demand in recent months, with long queues forming at security and many struggling to get their luggage off of airplanes.
More than 10,000 passengers are believed to have been impacted by last minute groundings at Britain's biggest airport yesterday.
Before lunchtime today 60 flights were cancelled, made up of 48 outbound and 15 arrivals.
“The global aviation industry is recovering from the pandemic, but the legacy of Covid continues to pose challenges for the entire sector as it rebuilds capacity," Mr Holland-Kaye continued.
"At Heathrow, we have seen 40 years of passenger growth in just four months. Despite this, we managed to get the vast majority of passengers away smoothly on their journeys through the Easter and half term peaks.
"This was only possible because of close collaboration and planning with our airport partners including airlines, airline ground handlers and Border Force.
“We started recruiting back in November last year in anticipation of capacity recovering this summer, and by the end of July, we will have as many people working in security as we had pre-pandemic.
" We have also reopened and moved 25 airlines into Terminal 4 to provide more space for passengers and grown our passenger service team.
“New colleagues are learning fast but are not yet up to full speed. However, there are some critical functions in the airport which are still significantly under resourced, in particular ground handlers, who are contracted by airlines to provide check-in staff, load and unload bags and turnaround aircraft.
"They are doing the very best they can with the resources available and we are giving them as much support possible, but this is a significant constraint to the airport's overall capacity. "
The airport boss admitted that it would still be busy despite the reductions and asked people to prepare their bags and Covid paper work ahead of security check-in.
He added: “By making this intervention now, our objective is to protect flights for the vast majority of passengers at Heathrow this summer and to give confidence that everyone who does travel through the airport will have a safe and reliable journey and arrive at their destination with their bags.
"We recognise that this will mean some summer journeys will either be moved to another day, another airport or be cancelled and we apologise to those whose travel plans are affected."
