Holidaymakers are having to pay for their overseas accommodation for a second time following the collapse of tour operator Sun4U.
The Civil Aviation Authority (CAA) warned that Sun4U booked travellers arriving overseas may find their airport transfers and booked accommodation unavailable to them.
The Birmingham-based company ceased trading on Wednesday night, with about 1,200 UK holidaymakers abroad, mostly in Spain.
Some of these are covered by the CAA-run Atol (Air Travel Operators' Licensing) scheme and will be able to carry on with their holidays and travel home.
But Sun4U also organised flight-only and accommodation-only trips which are not covered by the Atol arrangement.
Also, the CAA said that even if holidaymakers' trips are Atol-covered, "hotels and accommodation agents may require customers who are abroad on holiday to pay again for their accommodation".
The CAA said that Atol-covered holidaymakers would be able to claim this money back from the CAA.
The CAA advised those Atol-protected customers who find their airport transfer and accommodation has become unavailable to make their own arrangements and then send a claim to the CAA.
A spokeswoman said: "We understand there are around 1,200 people abroad and we are trying to find out just how many are Atol-protected.
"We are also trying to determine just how many people have forward bookings with Sun4U."