Travel firm Thomas Cook has collapsed after last-minute negotiations aimed at saving the 178-year-old holiday company failed.
The UK Civil Aviation Authority (CAA) said that the tour operator had 'ceased trading with immediate effect'.
With hundreds of thousands of holidaymakers abroad and many booked to travel in the upcoming months, we explain how you're protected, what to do next and how to claim a refund.
The Atol scheme protects people who buy package holidays that include a flight, through UK tour operators such as Thomas Cook.
So whether you've booked a holiday package or just a flight through the UK tour operator, Atol protection means you shouldn't lose money or be stranded abroad.
If your flight or holiday is Atol protected, you should have received an Atol Certificate by email or by post when you made a payment towards the booking.
Adam French, Which? consumer rights expert, said: 'Hundreds of thousands of holidaymakers affected by the collapse of Thomas Cook will be incredibly worried, especially if they're currently still on holiday and stranded abroad. The good news is that Atol protection will mean they will be flown back home free of charge.
'Customers who have already paid and booked for an upcoming holiday with the collapsed travel operator should also be entitled to a refund as part of the scheme.
'However, if you didn't book as part of a package you might not be Atol protected, but you may be able to claim the cost back through your travel insurance or credit card issuer - it will depend on your circumstances.'
Atol protection only applies to hotels and flights when booked as part of a package holiday. So, if you booked your flights and hotel separately, this won't be Atol-protected.
You should contact your card issuer, bank, or travel insurer for advice on whether you can claim a refund.
Your journey home will either be on a CAA-operated repatriation flight or on an existing flight with another airline depending on where you are.
The CAA will try to get you on a return flight as close to your original departure time as possible. Your costs for coming home will be covered.
Now Thomas Cook has gone into administration, your next steps are:
If you are currently abroad and due to return to the UK on or before 6 October 2019, the Civil Aviation Authority is working to arrange for all passengers whose flights have been cancelled to return on a new flight.
Where the CAA is not able to provide a new flight, you will need to book your own return flight but will be able to claim the cost of this back.
Non-Atol protected passengers flying back to the UK after 6 October 2019 will need to make their own new travel arrangements.
If you booked your flights through another travel company, you should contact them for advice or speak to your travel insurer or card issuer about making a claim.
If you are Atol protected, you will still be assisted by the CAA. Get in touch with the CAA closer to the time to confirm details.
If you're experiencing difficulties with your Atol-protected hotel, or your hotel is requesting payment from you, you can call the CAA call centre on +44 1753 330 330.
Please note that it may take the CAA a few days to secure new payment arrangements. Do not make a payment to your hotel unless instructed otherwise by the CAA team.
If you are not Atol protected, you are not entitled to make a claim for out of pocket expenses and additional nights of hotel accommodation under the Atol scheme, but you may be able to claim from your travel insurer, bank or your credit card issuer.
If you're due to travel with Thomas Cook in the future, here are your next steps:
The troubled tour operator tried to secure a £750m cash injection to see it safe over the winter 2019-20 season when bookings typically tail off.
Thomas Cook's chief executive, Peter Fankhauser, said the firm's collapse was a 'matter of profound regret'.
Commenting as the company entered compulsory liquidation, Mr Fankhauser also apologised to the firm's 'millions of customers and thousands of employees'.
If the company does go bust it would be the biggest-ever repatriation, with an estimated 150,000 Britons currently abroad on Thomas Cook holidays.
The government has chartered 45 jets to bring customers home and they will fly 64 routes today. The size of the fleet will make it temporarily the UK's fifth largest airline.
The collapse has put 20,000 jobs at risk, including 9,000 in the UK.