The Civil Aviation Authority (CAA) believes that 150,000 British holidaymakers are currently abroad and will need to be repatriated and has launched a major repatriation operation to fly home stranded holidaymakers.
The operation is almost twice the size of the repatriation effort required when Monarch went bust in October 2017.
-> Thomas Cook holidaymakers won't be left stranded says foreign secretaryIn that instance, the CAA put on 567 flights which brought almost 84,000 passengers back to the UK and the final cost to taxpayers was about £50 million.
The Department for Transport would not put a firm figure on how much the Thomas Cook operation will cost, but it is understood it could top £100 million.
Dozens of charter planes have been brought in from as far afield as Malaysia to assist with the mass airlift of Thomas Cook customers.
Boris Johnson has pledged to help holidaymakers stranded by Thomas Cook's closure.
The Prime Minister, speaking before the firm collapsed, told reporters on board the RAF Voyager travelling to New York for the United Nations General Assembly that his thoughts were with customers.
He said: "It's a very difficult situation and obviously our thoughts are very much with the customers with Thomas Cook, the holiday makers, who may now face difficulties getting home.
"We will do our level best to get them (travellers) home. There will be plans ready to deal with that if it's necessary."
Only holidaymakers with return flights booked within the next two weeks, between Monday and Sunday October 6, will qualify for a free flight home, as close as possible to their original return date.
On Monday September 30, one week into the repatriation process, the CAA will launch a service which will seek to process all refunds within 60 days of full information being received.
All future Thomas Cook holidays and flights are cancelled with immediate effect and the CAA’s repatriation programme will not include any outbound flights from the UK.