Heathrow asks airlines to stop selling summer tickets as it limits daily passengers to 100,000

London(CNN Business) Heathrow Airport is asking airlines to stop selling any more tickets for travel this summer, as it announces a cap on passengers allowed to fly from the airport.

The London airport will limit passengers to 100,000 a day until September 11, as it struggles to cope with high demand and a lack of resources.

CEO John Holland-Kaye announced the "difficult decision" in an open letter to passengers, saying "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... 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."

Many airlines have been working to reduce the number of passengers going in and out of Heathrow. But Holland-Kaye said Heathrow's latest forecast showed an excess number of seats had already been sold and therefore airlines needed to stop selling tickets now.

"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"

In 2018, the daily number of passengers going through Heathrow was nearly 220,000, split between arrivals and departures.

Lufthansa (DLAKY), which has already canceled thousands of departures from Frankfurt and Munich for the summer season, said it might make further adjustments to its schedule "for traffic peaks in August."

"Lufthansa has thus made a noticeable contribution to relieving the airports, including London Heathrow," a spokesperson told CNN Business.

The head of International Air Transport Association, the group representing global airlines, called Heathrow's travel restrictions "ridiculous."

"Airlines have been predicting stronger traffic than Heathrow has been predicting ... they clearly got it completely wrong," said Willie Walsh, director general of the IATA. "To tell airlines to stop selling -- what a ridiculous thing for an airport to say to an airline."

Walsh, the former CEO of British Airways owner IAG, added, "Heathrow are trying to maximize the profitability that they get from the airport at the expense of airlines."

Heathrow rejected Willie Walsh's comments.

"Aviation is under considerable pressure as demand ramps up — at Heathrow we've faced 40 years of growth in just four months — and what we need is collaborative working and investment in services to protect passengers, not ill-informed comments from retired airline bosses," a spokesperson said.

— Sharon Browne-Peter contributed reporting.