One airline reported a "network-wide failure" but NATS, the national air traffic controllers, said UK airspace was not closed but restrictions were in place on one of the busiest travel days of the year due to the UK bank holiday.

Fault may cause 'very high' level of delays - live updates

In a statement, it said: "We are continuing to work hard to resolve the technical issue. To clarify, UK airspace is not closed, we have had to apply air traffic flow restrictions which ensures we can maintain safety."

Read more:
What we know about system failure and how it's affecting flights

NATS earlier said: "Engineers are working to find and the fix the fault. We apologise for any inconvenience this may cause."

It did not give an estimate of how long it would take to fix the problem, or what had caused it.

European air traffic authority Eurocontrol warned of "very high" delays because of a "flight data processing system failure" in the UK.

Meanwhile, AirNav Ireland, the country's air traffic controller, said: "NATS is currently experiencing a technical issue that is restricting the capacity of UK airspace. This is resulting in significant delays for flights across Europe that are travelling to, from or through UK airspace.

"At this time, there is no certainty as to when flights will be back on schedule so passengers who are due to travel today should contact their airlines in the first instance to check if their flights are delayed."

Some 3,049 flights were due to depart from UK airports today, with another 3,054 flights were scheduled to arrive in the UK today - with around one million passengers on board.

The Liberal Democrats have called on the government to hold an urgent COBRA meeting while the Labour's shadow transport secretary said the fault was "extremely concerning".

Passengers flying to and from the UK have been told to expect severe delays of up to 12 hours.

BBC presenter Gabby Logan said she had been caught up by the issue.

She posted on X, formerly known as Twitter: "On a plane on the runway at Budapest airport. After almost three 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."

Sky's science correspondent Thomas Moore is one of the passengers stuck on a British Airways flight at Heathrow after flying back from San Francisco overnight.

Speaking to us from the plane, he said the aircraft actually landed ahead of the schedule - but since then they have been stuck on the tarmac.

"The pilots told us that we are in a queue for a gate because all the aircraft giving priority to land can't get away, so everything is stacking up now," he said.

"We have an update saying that they're trying to find space for all the aircraft that are coming in and having to wait, having to queue, because this is going to have a big impact.

"Looking out of the window, I can see aircraft movements.

"There are planes taking off, but what we are being told is that various aspects of computerised systems used by air traffic control aren't talking to each to other, so everything is having to be done manually.

"That means things are taking a lot longer than they would be.

"Normally this would be one of the busiest runways in the world, but it is looking very, very quiet."

Sky News producer Georgia Ziebart, who is stuck on a plane in Palma, Majorca, that is set for London Gatwick said passengers have been told all planes that were in the air at the time the systems went down have been diverted to other countries.

"We're on the tarmac in Palma, Majorca - we got on the plane an hour ago and shortly after we got on the plane, the pilot made an announcement to say it doesn't look like we'll be leaving for a while because there are issues with air traffic control across the whole of the UK.

"We've been sat on the plane now for about an hour, still haven't moved, still haven't had much information. But staff just said there's no point in us moving because we can't actually go anywhere.

"There are people who have been sleeping at the airport since yesterday so it's completely at capacity inside as well.

"There's a lot of children on board here, a lot of babies. It's a three-hour flight. I haven't got any food. They've come around and offered water to everyone but that's it at the moment."

Airports and airlines have warned customers that they may experience delays and urged those travelling today to check their flight details.

Ryanair and Aer Lingus were the first airlines to confirm several flight cancellations to and from Dublin and Cork airports.

Aer Lingus said: "Due to a failure of the UK air traffic control flight planning systems, which is outside of Aer Lingus' control, there are severe restrictions on all flights into the UK and flying over UK airspace today."

Ryanair said: "Due to another UK ATC [air traffic control] failure, Ryanair will be forced to delay/cancel a number of flights to/from the UK today, 28 Aug."

British Airways said it was "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".

Airports across the UK said they had been affected by the fault and were working to understand its full impact. Meanwhile, Dublin and Cork airports have both said they are experiencing delays to some flights.

London Gatwick and Newcastle airports said they were expecting cancellations.

The Eurostar said it would add an extra train from Paris to London at 8.43pm this evening after the airline disruption.