British Airways is urging customers to allow extra time at airports this morning due to system failures causing problems checking passengers in online and getting information to pilots.
Passengers have complained on social media about long queues, flight delays and the BA app crashing.
BA said it had been forced to revert to checking in passengers manually but it has had to delay some flights as a result.
It said it was experiencing problems at Heathrow, Gatwick and London City airports.
The Independent is reporting that, so far, 86 flighs have been cancelled at London Heathrow and 16,000 passengers have been affected.Others are experiencing two-hour delays. It said there were also some delays at Gatwick, where 10 flights have been cancelled so far.
In a statement, the airline said: "We are very sorry to our customers for the disruption to their travel plans.
"We are working as quickly as possible to resolve a systems issue which has resulted in some short-haul cancellations and delays from London airports.
"A number of flights continue to operate but we are advising customers to check ba.com for the latest flight information before coming to the airport, and to leave additional time.
"We are offering customers booked on short-haul services departing from Heathrow, Gatwick and London City today, the opportunity to rebook to another day."
Passengers can rebook flights for departures between August 8 and 13. Those whose flights have been cancelled are being offered refunds or alternative services.
BA said the issue was not a global outage, but a problem with two separate systems - one which deals with online check in, the other that deals with flight departures.
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