The UK's air traffic control system has suffered a 'huge network failure' on one of the busiest days of the year - causing chaos for travellers that could last days.
Britain's National Air Traffic Services (NATS) said it is experiencing 'technical issues' that had forced controllers to reduce the rate flights are able to land and take off.
This has led to scores of flights being delayed both in and out of the UK. One Briton returning from Lanzarote told MailOnline she had been told to expect a wait of at least 10 hours.
Travel expert Simon Calder said the system outage would cause 'misery' for passengers. Contagion has already spreading across Europe, including for flights leaving the Continent for the USA.
He said travellers should 'assume' their flight was operating normally and predicted airlines would be forced to pay out millions in compensation.
TV presenter Gabby Logan said she has been left stranded on the runway at Budapest Airport.
She wrote: 'After almost 3 weeks away from home I am hours from hugging my family. And have just been told UK airspace is shut. We could be here for 12 hours. So we sit on the plane and wait.'
NATS did not provide any further information about what caused it or how long it would take for UK airspace to return to normal.
Frustrated passengers on board a flight leaving Lanzarote to Newcastle - one of many that has been hit by today's disruption
Passenger Eric Parzianello is on a Delta flight from Amsterdam to Detroit, which has been delayed by the air traffic control outage
A long queue of passengers queueing outside the terminal in Mallorca following today's air traffic control outage
TV presenter Gabby Logan said she has been left stranded on the runway at Budapest A
Mr Calder told Sky News: 'There is very little slack in the system. It's going to be miserable. Meanwhile there and hundreds of planes up in the sky heading to the UK.
'What's going to happen to those aircraft, will some of them get down if they are in the vicinity of the airfield.
'Otherwise you will see planes held on the ground in places like Amsterdam or otherwise being diverted if they're on a longer flight. That would typically be to a continental airport or an Irish airport.'
Mr Calder said the shutdown would not cause safety issues because the system was 'designed to cope' with a shutdown and aircraft carried contingency fuel.
But he added: 'This is of course one of the busiest days of the year. There are hundreds of thousands of people flying into the UK, frankly this is the last thing anyone needs.
'It will at the very least have caused enough disruption for the system to be in disarray for certainly until the end of the day and possibly for a few further days ahead.'
The travel guru said air traffic controllers at Heathrow - the UK's busiest airport - be forced to reduce the frequency at which flights are able to land.
He explained: 'Normally you have flights landing typically every 90 seconds or so. They can switch away from the digital system and become much more analogue, bringing the aircraft in more manually. However, you are not going to be able to do it at the same rate.
'For Heathrow and Gatwick in particular there is so little slack in the system that it can cause problems. If you're reducing the flow rate coming in and keeping aircraft on the ground at those airports it will be a very difficult afternoon.'
At Stansted, Ryanair passengers told PA they had been told to wait at their gate until further notice.
One passenger, Danni - who is travelling back to the UK from Lanzarote - told MailOnline she had been told to expect a delay of at least 10 hours. This is a photo she took from her window
Today is one of the busiest days for air travel of the year, making the 'huge network failure' all the more disruptive. This is a map showing flights currently in the air across Europe
News of the shutdown was leaked out by the Scottish airline Loganair, which shared the news on Twitter.
It tweeted: 'There has been a network-wide failure of UK air traffic control computer systems this morning.
'Although we are hopeful of being able to operate most intra-Scotland flights on the basis of local coordination and with a minimum of disruption, north-south and international flights maybe subject to delays.
'If you are flying with us today, please check our website for the latest information about your flight before setting off for the airport.'
British Airways said in a statement: "We are working closely with NATS to understand the impact of a technical issue that is affecting UK airspace and will keep our customers up to date with the latest information."
NATS said: 'We are currently experiencing a technical issue and have applied traffic flow restrictions to maintain safety. Engineers are working to find and the fix the fault.
'We apologise for any inconvenience this may cause.'
An aviation expert shares a map showing the area that has been affected by the shutdown
Travel expert Simon Calder said the disruption would force Heathrow (pictured) to delay the frequency at which it allows flights to land
