Work is under way to bring home 110,000 Monarch Airline customers, while advice has been issued to the 300,000 people already booked up on breaks.
Here is what you need to do if you are one of those affected.
Monarch passengers abroad:
*What should I do now?
The Government has told Monarch customers to continue their holidays as planned and is organising special flights back for passengers who are affected at no cost to them.
At least 48 hours before you are due to return home you should visit the dedicated website 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 your new flight details.
Repatriation flights are for all passengers who purchased tickets with Monarch irrespective of their nationality.
*When will I be able to fly?
You should expect to be flown as close as possible to your planned departure dates, no earlier, and prepare for disruption to journeys.
Some passengers may need to speak with their accommodation provider in case they need to extend their stay.
*When should I go to the airport?
You should arrive at the airport more than three hours before your confirmed new flight as they will not be able to take earlier services.
Foreign Office staff will be available at affected airports to help any vulnerable British citizens with specific needs. Any Britons requiring additional consular support should visit gov.uk/world.
Can I check in online?
There will be no online check-in. You will be issued with a new flight and new boarding card and will not be able to check in with your old flight details.
*Will I have to pay for the flight?
Once on board the plane, you will be asked to provide details of your original Monarch booking.
If you are not Atol (Air Travel Organiser's Licence) protected, this will allow the Government to claim the cost of their replacement flight directly from your credit or debit card company.
You will not be asked to pay for your flight yourself.
*Do I have to pay for extra accommodation costs?
Passengers with Atol protection are entitled to reasonable accommodation and subsistence costs if they are delayed by more than four hours.
Those without Atol protection may be able to claim from credit card providers or insurers. Further information will be made available via the website.
The CAA will process your refund as quickly as possible - check the dedicated website for more information.
*How do I know if I am Atol protected?
Check if you have received an Atol certificate, or check with your travel agent.
*Will I be flown back to my departure airport?
You may be flown back to different UK airports, with coaches available to take you to your destination airport.
Monarch customers yet to travel:
Check the dedicated website or contact your travel agent if you booked with one.
They will be able to advise on whether you can change your holiday booking or are entitled to a refund or compensation. If you are not covered by the Atol scheme, you should also contact your credit card company or travel insurer.