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.
Mr Fankhauser said: "Despite huge efforts over a number of months and further intense negotiations in recent days we have not been able to secure a deal to save our business."
He added: "It has been my privilege to lead Thomas Cook. It is deeply distressing to me that it has not been possible to save one of the most-loved brands in travel."
Prime Minister Boris Johnson pledged to help stranded holidaymakers, but also questioned whether company directors were properly incentivised to "sort such matters out".
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."
Transport Secretary Grant Shapps said the company's collapse was "very sad news for staff and holidaymakers" and urged those affected to be "understanding with staff".
Mr Shapps has announced that the government and CAA has hired dozens of charter planes to fly customers home free of charge.
The emergency operation, codenamed Operation Matterhorn, is aiming to bring home Britons currently on holiday with the firm.
On Sunday, empty aircraft had already started to be flown overseas, ready to bring British tourists back to the UK on Monday.
One of the world's best known holiday brands, the business was founded in 1841 in Leicestershire by cabinet-maker Thomas Cook.