Here's what we know so far.

What is the fault?

At the moment NATS, the National Air Traffic Controllers, is calling it a "technical issue".

"Engineers are working to find and fix the fault," it said in a statement.

But it has said the airspace is "not closed".

They are reducing the number of aircraft that can take off land at a given time to manage flow.

"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."

What happens while they fix the fault?

Planes that are already in the air may be diverted to airports in Ireland or mainland Europe.

NATS said it has "applied traffic flow restrictions to maintain safety".

For passengers, that means delays and possible cancellations.

What's happening to flights?

Some planes will be able to land if they were already in the vicinity of the airfield, but otherwise flights will be held on the tarmac or diverted to airports, travel journalist Simon Calder told Sky News.

"There are hundreds of planes up in the sky heading to the UK," he said - meaning hundreds of thousands of passengers could be affected.

The August bank holiday is one of the busiest days of the year for air travel.

Mr Calder said some flights would be able to land as air traffic controllers switched from digital to "much more analogue" systems, but the rate would be much slower.

How long could the disruption last for?

Even if the issue was fixed quickly, the system would likely be in "disarray" for at least the rest of Monday and possibly longer, Mr Calder said.

Aviation analyst and former BA pilot Alastair Rosenschein told Sky News it was "very difficult" to say when things would be back to normal.

But he offered some reassurance, saying the disruption looked to be "delays of hours rather than anything longer".

He said the issues appeared to be a case of "patchy failure rather than total shutdown".

What airports are reporting disruption?

Airlines have released similar statements while they assess the likely impact of the situation.

Dublin Airport said there were some delays to flights in and out of the airport due to Air Traffic Control issues in the UK

"We advise all passengers due to travel today to check the status of their flight with their airline in advance of travelling," it said.

Ireland's Cork Airport has said the air traffic control issues are causing delays for some departing and arriving flights.

Gatwick Airport has warned cancellations are "likely" and it is "seeing delays", after earlier saying flights were running as normal.

Luton Airport says: "We are aware of an air traffic control issue affecting UK airspace, resulting in disruption to flights.

"We are working with all relevant authorities to understand the full impact of the issue and when normal operations are likely to resume.

"Passengers should check with their airline for the latest status of their flight."

Birmingham Airport says: "We are working to understand the full impacts of the air traffic control failure across UK airspace, and the likely timescales for to be resolved.

"Customers due to fly in or out of BHX are advised to keep a close eye on the latest information from their airlines, as well as live flight data on our website and on screens in our terminal."

Heathrow Airport says it is "working closely" with NATS and other airports to "minimise the impact" on passengers. '

Stansted Airport says it is "fully operational" and is not experiencing any issues.

Liverpool John Lennon Airport warned of disruption, saying: "Whilst aircraft will be loaded on time, some flights may be subject to a delay."

Newcastle International Airport says passengers face "disruption" and "some cancellations".

Edinburgh Airport asks passengers "not [to] come to the airport before checking with their airline on the status of their flight".

What are airlines saying?

Scottish airline Loganair, one of the first to report disruption, said there was a "network-wide failure of UK air traffic control computer systems" on Monday 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," the airline posted on X, formerly known as Twitter.

It urged passengers to check its website for updates.

British Airways said it is "working closely with NATS to understand the impact of a technical issue that is affecting UK airspace".

What should I do if I'm travelling?

If you're due to fly in or out of the UK today, check the status of your flight with your airline - this is what each airport and airline is currently advising.

If you're already at the airport, keep an eye on display screens.

This article will continue to be updated with all the latest information you need if you're due to travel.

You can also follow our live blog which will have updates on what's going on.

What are my rights if my flight is delayed?

Disruption to air traffic control counts as an "extraordinary circumstance".

That means it is out of the airline's control - so they don't have to offer compensation.

But if your flight is delayed by two hours or more, airlines must offer assistance under UK law.

They have a duty to provide:
• two free phone calls, faxes or emails (often by refunding the cost of your calls)
• free meals and refreshments appropriate to the delay (often in voucher form)
• free hotel accommodation and hotel transfers if an overnight stay is required.

Sometimes airlines may be unable to provide assistance to all passengers, particularly where staff are stretched - which could be the case with the mass delays seen today.

If this is the case, the Civil Aviation Authority says passengers should organise their own assistance and claim the cost back later.

"If you end up paying for things yourself, keep every receipt and do not spend more than is reasonable," its website says.

It cautions airlines are unlikely to reimburse for alcohol or luxury hotels. Some airlines provide guidelines on what counts as reasonable expenses.