British tour operator Goldtrail, which specialises in holidays to Greece and Turkey, has collapsed, leaving thousands of holidaymakers abroad, the Civil Aviation Authority said.
The Surrey-based company, which mainly sold its holidays through travel agents, went into administration on Friday.
It had about 16,000 customers on holiday abroad at the time of its collapse, media reported.
The CAA said it was arranging to fly holidaymakers home at the end of their holidays, with flights from Turkey expected to operate as normal but passengers travelling back from Greece likely to face changes to their journey.
Customers yet to leave Britain were advised to check with their travel agent as no more outbound Goldtrail flights will operate and existing tickets will no longer be valid.
The company's collapse comes just ahead of the busiest holiday period of the year, with most British schools closing for the summer break at the end of next week.