British Airways customers have described the "total chaos" as they try to jet off for their summer holidays on Wednesday morning (2 August) after the airline's computer system crashed.
Passengers at Heathrow, Gatwick and City airports faced delays and long queues as the airline said there were "some issues" with its check-in system.
Some travellers said they missed their flights due to the IT meltdown, which comes just two months after the airline was forced to cancel more than 670 flights due to another systems failure.
Passengers who did manage to make it on to their flight complained of then having to wait for a significant amount of time on the runway.
Customers took to Twitter to express their anger at British Airways.
One holidaymaker at Heathrow wrote on Twitter: "[Terminal 5] is absolute carnage with systems down!! Very unhappy customers left right and centre. #stillqueuing."
Another wrote: "Missed my 7:15 flight because BA couldn't print my boarding pass due to IT failure. Didn't even have baggage to check in."
A third said: "Total chaos at Heathrow."
British Airways, whose owner IAG last week reported a 37% jump in first-half profits, apologised for the fault and said the problem has now been resolved.
A spokesperson for BA said: "Customers are being checked in as normal after an earlier problem was resolved. We are sorry for the temporary check-in problems, which caused some delays for our customers first thing this morning.
"This issue is now resolved and our staff are working flat out to help customers get away on their holidays."
The delays for passengers trying to leave the country came as British holidaymakers flying back to the UK faced long queues at European airports due to tighter EU border checks.
Some travellers waited in terminals for longer than their actual flight time as they tried to get through immigration control.