The tour operator Thomas Cook has ceased trading with immediate effect after failing in a final bid to secure a rescue package from creditors.
More than 150,000 British holidaymakers are currently abroad and will need to be repatriated as a result of the 178-year-old firm's collapse, the Civil Aviation Authority (CAA) said.
The CAA said in a statement: "All Thomas Cook bookings, including flights and holidays, have now been cancelled. There are currently more than 150,000 Thomas Cook customers abroad, almost twice the number that were repatriated following the failure of Monarch.
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"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."
The group's four airlines will be grounded and its 21,000 employees in 16 countries, including 9,000 in the UK, will be left unemployed.
The company also operated around 600 UK high street stores.
The CAA said the Government had asked it to launch a repatriation programme over the next two weeks, starting on Monday and running to Sunday 6 October, to bring Thomas Cook customers back to the UK.
The CAA statement said: "Due to the unprecedented number of UK customers currently overseas who are affected by the situation, the Civil Aviation Authority has secured a fleet of aircraft from around the world to bring passengers back to the UK with return flights.
"Passengers in a small number of destinations may return on alternative commercial flights, rather than directly through the Civil Aviation Authority's flying programme. Details and advice for these passengers are available on the dedicated website.
"Due to the significant scale of the situation, some disruption is inevitable, but the Civil Aviation Authority will endeavour to get people home as close as possible to their planned dates. This will apply to both Atol protected passengers and those who are not protected.
"Customers currently overseas should not travel to the airport until their flight back to the UK has been confirmed on the dedicated website.
"Thomas Cook customers in the UK yet to travel should not go to the airport as all flights leaving the UK have been cancelled."