BBC News: Thomas Cook has collapsed after last-minute negotiations aimed at saving the holiday firm failed.
The UK Civil Aviation Authority (CAA) said the tour operator has “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”.
The tour operator’s failure puts 22,000 jobs at risk worldwide, including 9,000 in the UK.
Transport Secretary Grant Shapps said the tour operator’s collapse was “very sad news for staff and holidaymakers”.
He urged holidaymakers to be “understanding with staff” amid the “enormous” task of bringing people home.
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, will aim 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 home on Monday.
The CAA said in a statement: “All Thomas Cook bookings, including flights and holidays, have now been cancelled.
“We know that a company with such long-standing history ceasing trading will be very distressing for its customers and employees and our thoughts are with everyone affected by this news.”
All customers currently abroad with Thomas Cook who are booked to return to the UK over the next two weeks will be brought home “as close as possible” to their booked return date, a statement said.
Flights will start operating from Monday, with details of each flight to be posted on a dedicated website as soon as they are available.
Customers have been urged not to cut short their holiday or go to the airport without checking the website for more information about their return journey.
The UK CAA is also contacting hotels accommodating Thomas Cook customers, who have booked as part of a package, to tell them that the cost of their accommodation will be covered by the government, through the Air Travel Trust Fund/ATOL cover.
Thomas Cook had secured a £900m rescue deal led by its largest shareholder Chinese firm Fosun in August, but a recent demand from its lending banks to raise a further £200m in contingency funding had put the deal in doubt.
The holiday company had spent all Sunday in talks with lenders trying to secure the additional funding and salvage the deal, but to no avail.
It had also asked the government for financial aid, a solution also urged by Labour and union groups.
But on Sunday Foreign Secretary Dominic Raab told the BBC the government did not “systematically step in” when businesses went under unless there was “a good strategic national interest”
Customers on a package holiday have Atol protection – a fund paid for through industry levies – which will cover the cost of their holiday and repatriation.
Thomas Cook has blamed a series of issues for its problems including political unrest in holiday destinations such as Turkey, last summer’s prolonged heatwave and customers delaying booking holidays because of Brexit.
But the firm has also faced fierce competition from online travel agents and low-cost airlines.
In addition, many holidaymakers are putting together their own holidays and not using travel agents.
What are your rights?
If you are on a package holiday you are covered by the Air Travel Organiser’s Licence scheme (Atol).
The scheme will pay for your accommodation abroad, although you may have to move to a different hotel or apartment.
Atol will also pay to have you brought home if the airline is no longer operating.
If you have holiday booked in the future you will also be refunded by the scheme.
If you have booked a flight-only deal you will need to apply to your travel insurance company or credit card and debit card provider to seek a refund.
When Monarch Airlines collapsed in 2017, the government organised to bring home all the stranded passengers, whether they were covered by Atol or not.