Travellers were hit with fresh misery Monday morning as Heathrow cancelled 61 flights.
Some passengers were also experiencing delays at Stansted, and were asked to wait in a satellite terminal to help ease congestion.
Heathrow said it had cancelled more than 60 flights because it was expecting higher demand than the airport could handle safely amid widespread staff shortages.
A spokesman for the airport said: “We are expecting higher passenger numbers in Terminals 3 and 5 today than the airport currently has capacity to serve, and so to maintain a safe operation we have asked some airlines in Terminals 3 and 5 to remove a combined total of 61 flights from the schedule.
“We apologise for the impact to travel plans and we are working closely with airlines to get affected passengers rebooked onto other flights.”
Among the flights showing as cancelled on Heathrow’s departures board on Monday included several British Airways flights, to destinations such as Zurich, Gibraltar and Glasgow.
Also grounded were Virgin Atlantic flights to Austin and Delhi, while Air France cancelled a flight to Paris.
Airport officials said nearly six million passengers had passed through in June alone, taking the total to 25m in the first six months of the year.
They said this meant the airport has seen 40 years of passenger growth in just four months.
It said it would be reviewing the schedule changes made by airlines and apologised to those hit by the recent travel chaos.
“However, despite our best efforts there have been periods in recent weeks, where service levels have not been acceptable, with long queue times, delays for passengers with reduced mobility, bags not travelling with passengers or arriving late, and we want to apologise to any passengers who have been affected by this,” said a spokesperson.
The fresh airport misery comes as the train drivers’ union, ASLEF, prepares to announce its decision on whether to strike.
It could lead to even greater disruption than last month’s strike by the RMT union.