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As many as 80 per cent of flights leaving the UK are delayed following a ‘huge failure’ of the national air traffic control system – causing chaos for tens of thousands of holidaymakers on one of the busiest days of the year.

Britain’s National Air Traffic Services (NATS) said it is experiencing ‘technical issues’ that have forced controllers to switch from an automatic system for landing and dispatching flights to a manual one.

‘Flight plans are being input manually which means we cannot process them at the same volume, hence we have applied traffic flow restrictions,’ NATS said. A spokesman told MailOnline there was ‘nothing to suggest a cyber attack’.

The majority (78%) of flights leaving Heathrow are currently delayed, according to Flight Radar data from 1.45pm, compared to 74% at Gatwick, 81% at Manchester and 86% at Bristol.

According to analaysis carried out by Cirium, 232 flights have been cancelled departing UK airports so far today – equivalent to around 8% of all departures.

Meanwhile 271 flights have been cancelled arriving into UK airports so far today – equivalent to around 9% of all arrivals.

Britons returning from Tenerife told MailOnline they had been told to expect a wait of at least 12 hours.

Michele Robson, who used to work in air traffic control, said that it was ‘unusual’ for failures to last this long. As a result, ‘nobody really knows at this point how long it’s going to take,’ she told BBC Radio 4.

Travel expert Simon Calder said the issue would cause ‘misery’ but added that holidaymakers should ‘assume’ their flight was operating normally unless they were told otherwise.

Contagion from the issue has already spread across Europe, causing delays for some flights leaving the Continent for the USA.

TV presenter Gabby Logan said she has been left stranded on the runway at Budapest Airport while returning from the World Athletics Championships. 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.

Former air traffic controller Michele Robson told BBC Radio 4’s World at One programme: ‘There was a flight planning system failure this morning which affected both centres in the UK.’

Speaking from Jersey Airport while waiting to fly to London, she said: ‘Now they have enough data for four hours for controllers to work normally. After that point, they have to go manual which means that they work at a much slower rate so they can handle far less aircraft.

‘So it looks like there’s been what they would call a zero rate put on, where it means that no aircraft can take off inbound to the UK or probably outbound. It would generally be them trying to land things that were already in the air.

‘So at the moment, we’re just sitting here with no definite takeoff time.’

The Liberal Democrats called on the Prime Minister to convene a Cobra meeting over the issue

Transport spokesperson Wera Hobhouse said: ‘Rishi Sunak and his ministers need to get a grip on this issue urgently and hold a Cobra meeting.

‘Millions of holidaymakers could be facing huge disruption in the coming days due to this fault and we can’t risk this Government being missing in action yet again.

‘Brits need to know that the Government is doing all it can to make sure people aren’t hit with major delays and disruptions in the coming days.’

Simon Calder warned the situation would be ‘miserable’.

‘There is very little slack in the system and there are hundreds of planes up in the sky heading to the UK,’ he told Sky News.

‘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 said they had been told to wait at their gate until further notice.

Beth McKendrick, 26, is currently sitting on the floor of Palma airport with bride-to-be Elizabeth Spadea, 25, having been at the Spanish city for a hen do.

She is trying to get back to Stansted and has described the situation as a ‘hen do from hell’.

‘When we attempted to jet-set back to the UK, we found ourselves smack dab in the middle of an aviation nightmare,’ she told MailOnline.

‘The entire air traffic control system in the UK has decided to take an unexpected bank holiday, leaving us stranded delayed for days on end. You might be thinking, ‘Surely, Ryanair saved the day!’ Think again. Not a peep of communication, let alone compensation, from the airline. Classic.’

Other Brits trapped abroad include Melissa Littlewood, who was due to be flying back to London after a family holiday at Disneyland.

She said: ‘We are on our way back to London from Disneyland and have just learnt we have a minimum of seven hour delay.

‘Waiting to speak to someone from BA [British Airways] to find out what options we have but won’t hold out breath.

‘Two tired and unhappy children with two adults not sure what to do next.’

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.

Simon Cullen is currently sitting on a BA plane waiting to get home and described his situation as a ‘true nightmare’.

He said: ‘All sitting on the plane waiting to get home. Also one passenger with a nut allergy onboard who is on and off the plane and freaking out the passengers.

‘No idea what’s going on apart from what we see on the news. Also BA business class is terrible with no room at all. Have a leg injury and we are squashed in like sardines.

‘Will never fly with them again and this is now turning out to be a true nightmare and we hope we get home to the UK.’

Passenger Eric Parzianello is on a Delta flight from Amsterdam to Detroit, which has been delayed by the air traffic control outage

Matthew Tully is stuck at Heathrow waiting on a flight to New York.

‘They just announced everything is delayed with no time for possible takeoff,’ he said.

Others have complained of having no air-conditioning at some airports abroad with the searing heat making it even more uncomfortable.

Mark Leah said: ‘Delayed 4 hours (hopefully not longer), stuck in a holding area to aircraft with no aircon at Porto Santo airport.

‘Just told the LGW flight which departed at 1055am is returning.’

Meanwhile, Rob Miall has boarded a TUI flight from Split Croatia to Manchester.

‘Just been told to wait on board for at least an hour,’ he said. ‘It’s getting very hot on here.’

Vladi Ibberson is currently stuck at Sofia airport in Bulgaria.

‘Two flights for the UK (Gatwick and Luton) are stuck with no information and 37C heat. Wish us luck.’

Susan Gilchrist is currently at Faro airport and said: ‘We were due to take off at 11.25am and sat at the gate for 40 mins, but have now been moved to a remote location.

‘We’ve had no update from this side but a friend is on an easy jet flight also at Faro and has been told to expect a six hour delay.’

News of the shutdown was leaked out by the Scottish airline Loganair, which shared the news on Twitter.

Loganair 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 may be 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 fix the fault.

‘We apologise for any inconvenience this may cause.’

Jet2 said: ‘Please be aware that UK Air Traffic Control are reporting a significant failure of their systems resulting in significant delays to all flights departing and returning to the UK. Please see our website for further updates.’