Heathrow said it may ask airlines to axe more flights should previous schedule reductions not reduce disruption to a 'satisfactory' level.
The airport confirmed another 60 flights had been cancelled today.
British Airways, Virgin Atlantic and Air France were among the airlines affected, with the airport warning more could be forced to scrap flights.
“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,” an airport spokesperson said.
“We apologise for the impact to travel plans and we are working closely with airlines to get affected passengers rebooked on to other flights.”
Heathrow handled nearly 6mln passengers in June, bringing its total to 25mln in the first half of the year but cancellations have now become almost a daily occurrence.
The head of the London travel hub apologised to passengers for disruptions in recent weeks but did not rule out further delays given unprecedented demand.
Attempts to cope with "40 years of growth in just four months" led to long queues, delays and cancellations at Heathrow, according to the airport's chief executive John Holland-Kaye, who said, "we want to apologise to any passengers who have been affected by this."
Last month, the government and Civil Aviation Authority ordered airlines to ensure their timetables are "deliverable" as they couldn't meet demand during the Platinum Jubilee school holidays.
Thousands of travellers have been affected by travel chaos this summer caused by staff shortages at the airport, with the CEO stating more flight cancellations might be required in the coming months as airlines try to solve the problem.
"We have already seen times recently when demand exceeds the capacity of the airport, airlines and ground handler," he said in a statement.
"We will review the schedule changes that airlines have submitted in response to the government's requirement to minimise disruption for passengers this summer and will ask them to take further action if necessary. We want everyone who is travelling through Heathrow to be confident that they will have a safe and reliable journey."
The airport's problems will be exacerbated by a planned strike later this month from July 21 to July 24, set to occur during the first weekend of the summer holidays.