British Airways (BA) has apologised after IT issues caused dozens of flights in and out of Heathrow Airport to be cancelled.
A number of short-haul flights were cancelled on Wednesday evening and Thursday morning after an incident involving a "supplier IT system".
It added to earlier disruptions on Wednesday caused by a fire alert at Heathrow's air traffic control tower.
BA said the IT issue had been resolved but knock-on delays remained.
Some passengers said they had been delayed by almost 15 hours.
BA said: "An issue with a supplier IT system was resolved overnight. We are doing everything we can to keep any knock on disruption to our services to a minimum.
"We have apologised to our customers for the delay to their travel plans."
The carrier, owned by IAG, suffered a large computer system failure in May 2017, caused by a power supply issue near Heathrow, which stranded 75,000 customers over a busy holiday weekend.
Its chief executive said at the time it would take steps to ensure such an incident never happened again
Customers have tweeted their frustration at the recent disruption.
The @British_Airways /Heathrow systems problems are continuing to have an impact across Europe this morning.— Chris Browning (@chris_browning) July 19, 2018
Passengers who were supposed to fly from @Schiphol to @HeathrowAirport at 2115 last night have been delayed almost 15 hours until 1200 today. #flightdelay @BBCNews pic.twitter.com/rgsdUxSEfs
@British_Airways unbelievable flight cancelled due to an IT issue, whole day of a family holiday delayed ! Your app has just sent me my boarding pass for the cancelled flight BA seriously awful !— Basia Szumska-Hare (@basiacx) July 19, 2018
@British_Airways Why is BA1305 leaving before last nights scheduled BA1317 from Aberdeen to Heathrow? Shouldn’t people delayed for over 12 hours be prioritised?— Curtis (@Curtis97464962) July 19, 2018