Frustrated British Airways passengers are facing "total chaos" due to a "worldwide" computer failure that has grounded planes.
Holidaymakers - including parents with babies - have been stuck on board planes for hours due to severe disruption at airports across the UK, including Heathrow, Gatwick and Glasgow, and in Italy.
One passenger stuck on a flight from Newcastle said the pilot told those on board that it was a "global" failure.
Holidaymakers told how their getaways have been thrown in doubt and some passengers became so frustrated they just walked off planes and gave up.
BA confirmed it was experiencing "systems problems" which were affecting the check-in process - including its app - and delaying flight departures.
The airline is using back-up and manual systems to try and cope with the failure, but it was unknown when the problem would be resolved and operations would return to normal.
Passengers said the system used to generate load sheets for flights had failed. The same system failed and caused travel disruption last month.
Zac Lennie was stuck on a plane with his children for more than an hour with no idea when they would leave Gatwick.
He told Mirror Online: "We arrived at 4.30am for a 6.45am flight to massive queues. We were told there was a system failure.
"It took an hour and 15 minutes before we got through security and we had to go straight to the gate.
"They boarded us as normal, but 15 minutes after we were due to take off we were told the systems had gone down and we were awaiting paperwork.
"Thirty minutes later we were told the same. Now we’ve just been told we have our paperwork and we will possibly get to take off if we can get a slot, but we're currently on standstill with the plane door open.
"The staff are good, they're in a s*** position. There are a lot of families on my flight. Kids are getting very restless."
Another passenger, Michael, was stranded on a plane at Newcastle Airport while en route to Slovenia with a bride and groom who are getting married on Saturday.
He said: "The cabin crew on board have said it's a system failure in regards to the load sheets to allocate baggage and weight distribution for the trim.
"One cabin crew said the system had been down since they arrived for duty approximately 5.15am, but still boarded the passengers.
"The captain said over the intercom it is a global failure and no aircraft were moving due to the essential data required.
"We're two hours and ten minutes past our departure time. No movement here."
He said flight attendants were giving updates and handing out drinks, but passengers' bags had not yet been loaded.
Liz Waugh tweeted a photo while she had been stuck on a BA plane in Bologna, Italy, for almost an hour.
She wrote: "Delayed in Bologna an hour for same reasons."
Andy Hursey and his friends had surprised a pal with a trip to Gibraltar for his 50th birthday, but they weren't sure if they would make it there at all while stranded at Heathrow.
Speaking from on board the plane, Mr Hursey, said: "Some people who are flying to Gibraltar and back today have got off.
"We checked in no problem and we all got on board and the baggage was on, but 10 minutes before departure it was announced that the system had crashed.
We've been sitting here waiting since 6.50 this morning.
"We're going to Gibraltar for my mate's 50th birthday. It looks like we're going to lose today. It's just frustrating.
"The pilot has come out and spoken to us a couple of times.
"It's a bit upsetting. We're going for two days so hopefully we will get out of here today.
"If we have to leave tomorrow I'm not sure we will actually go.
"They said it's a worldwide glitch with the system."
Author David Hewson tweeted a photo while he was stuck in a long queue at the check-in desk at Gatwick.
He wrote: "Major computer failure with British Airways at Gatwick this morning.
"This is the queue for the queue at check in.
"Currently stuck on ground cos pilot can't get data.
"Set off early if you have a flight."
He added: "Stuck on the ground at Gatwick because your systems are down and pilots can't get weight and balance.
"Do you really have no idea how long this will go on?"
A passenger flying from Heathrow to Delhi had been stuck on board for hours.
He wrote: "@British_Airways sitting in BA256 for more than two and half hour, flight not taking off from Delhi, any updates when will it take off."
And another wrote: "Total chaos at Gatwick. System failure?"
Passengers trying to use the airline's mobile app said they were receiving an "error" message.
A British Airways spokesperson said: "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.
"We are offering customers booked on short-haul services departing from Heathrow, Gatwick and London City today, the opportunity to rebook to another day.
"We are encouraging customers to check ba.com for the latest flight information, and to allow additional time at the airport."
It comes as rush-hour commuters and holidaymakers face delays and cancellations after a fire broke out next to railway tracks near a major station.
Southern and Gatwick Express train services are not running to or from London Victoria.
Trains were cancelled after a fire started between Victoria and Clapham Junction, with disruption expected to continue until around 10am, National Rail said.