Online long-haul travel company Dreamticket has ceased trading following the collapse of parent company Selsdon Travel.
The Civil Aviation Authority has been called in to protect customers who had booked trips with the Croydon-based firm.
It said the company was licensed to carry 17,000 passengers a year. At the time of its collapse, it had 600 customers overseas and 4,000 advance bookings.
Dreamticket sold holidays on scheduled flights to the Indian Ocean, Middle East, Far East, Africa and the Caribbean.
The CAA is making arrangements for customers abroad to complete their holidays and return to the UK and to fully refund those with forward bookings.
Passengers currently overseas are advised to go to the airport as ticketed and return to the UK. They may be asked to repay their hotel bills, in which case they must obtain receipts and claim the money back from the CAA when they return home.
Customers who are abroad and experiencing difficulties should call the CAA on 0044 161 444 5810.
Customers who were due to travel at a later date should go to the CAA website for advice about refunds, or call 08444 933037.
By Linsey McNeill