Monarch has ceased trading with immediate effect, with around 110,000 of its customers overseas.
A further 300,000 future bookings, affecting 750,000 passengers, have been cancelled.
If you’ve booked with the airline, here is the latest information on what to do and what you can expect:
:: What if you’re already abroad?
Those who are abroad and due to return to the UK on or before 15 October will be flown home, at no extra cost, on more than 30 aircraft chartered by the Civil Aviation Authority (CAA) on behalf of the Government.
At least 48 hours before they are due to return home, passengers should visit monarch.caa.co.uk, call the helpline on 0300 303 2800 if calling from the UK (or +44 1753 330330 if calling from overseas) to confirm their new flight details.
All passengers with Monarch tickets back to the UK will be flown to the UK airports, irrespective of their nationality.
There will be no online check-in.
Vulnerable passengers, such as unaccompanied minors, will be given priority on these flights, while family groups will be kept together.
Video: ‘Mum was in tears’: Monarch passengers hit
:: Do you need to come home now?
Holidaymakers have been told they do not need to cut their trip short and will be able to fly home at the end of their holiday as planned.
New flights will be as close as possible to passengers’ original return date and time, although delays to holidaymakers’ scheduled return are described as “unavoidable”.
If a passenger’s new flight departs more than four hours after their original was due to leave, they will be covered for “reasonable expenses” by the CAA for food and drink, phone charges and parking as long as their trip is ATOL (Air Travel Organiser’s Licence) protected.
Claims can be made using itemised receipts on holidaymakers’ return to the UK.
Those departing the day after their original return date will be able to stay in their existing accommodation or an alternative.
If airport transfers were included as part of a holiday booking, they will be rearranged to meet passengers’ new flights, with information provided in-resort.
Those who are flown back to a different UK airport than booked will be given transport, at no extra cost, to the airport they were originally due to fly in to.
Those not ATOL protected will not be able to claim for additional expenses to the CAA but may be able to claim from insurers or card issuers.
If protected, an ATOL certificate will have been issued via email or post as soon as a payment for a booking was taken.
Video: CAA chairwoman: Not right to say people are ‘stranded’
:: What if you’re due to return after 15 October?
Holidaymakers due to return on or after 16 October will not automatically have their flights rebooked unless their trip is ATOL protected.
Those who are not ATOL protected and due to fly home on or after 16 October will need to book new flights themselves and chase up a possible refund with their card issuer or insurer.
:: What about your accommodation?
The CAA are working to ensure those whose ATOL protected holiday bookings included accommodation will be able to stay in the same accommodation, including the same provision of meals, for the duration of their trip at no extra cost.
Those who are required to make further payments for accommodation are told to pay and then claim back the money from the CAA on their return to the UK, using a receipt with a full breakdown of charges.
Image: Part of a notice handed out to passengers at Gatwick Airport
:: What if you’re yet to travel?
All future holidays and flights booked with Monarch have now been cancelled with passengers told not to go to the airport as flights will not be operating.
:: How do you claim compensation for cancelled flights?
Those who booked flights directly with Monarch Airlines on or after 15 December last year will not be ATOL protected and will not be able to claim a refund from the CAA.
They should instead contact their card issuer or insurer.
Those who booked with Monarch Airlines on or before 14 December 2016 and received an ATOL certificate stating their flight is protected with First Aviation are able to claim, with the CAA currently making arrangements for refunds.
Holidays booked directly with Monarch Holidays will also be protected.
Those who arranged their trip through another travel company or agent should contact them directly over refunds or alternative bookings.