Thomas Cook has collapsed after last-minute negotiations aimed at saving the 178-year-old holiday firm failed.
The UK Civil Aviation Authority (CAA) said the tour operator had “ceased trading with immediate effect”.
It has also triggered the biggest ever peacetime repatriation, aimed at bringing more than 150,000 British holidaymakers home.
Peter Fankhauser, Thomas Cook’s chief executive, 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”.
The tour operator’s failure puts 22,000 jobs at risk worldwide, including 9,000 in the UK.
One of the world’s best-known holiday brands, the business was founded in 1841 in Leicestershire by cabinet-maker Thomas Cook.
BBC transport correspondent Tom Burridge said 16,000 holidaymakers were booked to come back on Monday. Authorities hope to get at least 14,000 of them home on chartered flights.
The government has chartered 45 jets to bring customers home and they will fly 64 routes on Monday, in an undertaking dubbed Operation Matterhorn. The size of the fleet will make it temporarily the UK’s fifth-largest airline.
Operators including easyJet and Virgin have supplied some aircraft, with jets coming from as far afield as Malaysia.
Customers can visit the CAA’s special Thomas Cook website. Those scheduled to return to the UK within the next 48 hours or who are having problems with their accommodation or need special assistance can ring 0300 303 2800 in the UK or +44 1753 330 330 from abroad.
