Air passengers are facing significant delays to their flights after a "technical issue" hit the UK air traffic control system.
The National Air Traffic Services said on Monday afternoon that the glitch had been fixed, however hundreds of flights were cancelled.
The problem emerged on one of the busiest days of the year for flights, when many people were set to return from long weekend trips abroad.
If you do find your flight is affected, what are your rights?
If my flight is cancelled, can I get a refund or another flight?
If your flight is covered under UK law, your airline must let you choose between either getting a refund or an alternative flight, regardless of how far in advance the cancellation was made.
You can get your money back for any part of the ticket you have not used. So if you booked a return flight and the outbound leg is cancelled, you can get the full cost of the return ticket refunded.
If you still want to travel, your airline must find you an alternative flight. If another airline is flying to your destination significantly sooner, or there are other suitable modes of transport available, then you have a right to be booked onto that alternative transport instead.
What about food and accommodation?
If you are stuck abroad or at the airport because of a flight cancellation, airlines must also provide you with other assistance until you are able to fly to your destination.
A reasonable amount of food and drink (often in the form of vouchers)
A way for you to communicate (often by refunding the cost of calls)
Free accommodation, if you have to stay overnight to fly the next day
Transport to and from the accommodation.
If your airline is unable to arrange assistance, you have the right to organise this yourself and claim back the cost later. In this case, the Civil Aviation Authority advises people to keep receipts and not spend more than necessary.
What if my flight is delayed?
You are entitled to the same assistance if your flight is delayed by more than two hours.
You may also be able to claim compensation if your flight arrives at its destination more than three hours late. Again, the amount is based on how far you are flying.
If you are delayed by more than five hours and no longer want to travel, you can get a full refund.
What if I booked a package holiday?
If you booked a package holiday with a company that is an Abta member and your flight is cancelled, you are entitled to a suitable alternative flight or a full refund.
Can I claim extra compensation?
Disruption caused by things like airport or air traffic control employee strikes, bad weather or other "extraordinary circumstances" are not eligible for extra compensation.
However, in other circumstances - which are the airline's fault - you have a number of rights under UK law if you are flying from a UK airport on any airline, arriving at a UK airport on an EU or UK airline, or arriving at an airport in the EU on a UK airline.
What you are entitled to depends on what caused the cancellation and how much notice you were given.
If your flight is cancelled with less than two weeks' notice, you may be able to claim compensation based on the timings of the alternative flight you are offered.
The amount you are entitled to also depends on how far you were travelling. For example:
For flights under 1,500km, such as Glasgow to Amsterdam, you can claim up to £220 per person
For flights more than 3,500km, such as London to New York, you can claim up to £520 per person
What if flight delays mean I'm late back to work?
Airlines will not refund you for loss of earnings. They are only responsible for covering direct costs - such as hotel rooms, meals and alternative flights - but they are not obliged to cover consequential losses.
Travel insurance policies will not usually cover loss of earnings.
If you think you're going to be late back at work because of flight delays, you have a responsibility to contact your employer to let them know you won't be back as planned, legal experts say.
You should agree with your employer over how to categorise the absence; for example, using more annual leave or time banked in lieu. Taking unpaid leave could also be an option.
Employers have no legal obligation to pay employees who are absent in this situation, experts say, unless it is stated in their contract.
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