Holidaymakers are having to pay for their overseas accommodation for a second time following the collapse of tour operator Sun4U.
And the Civil Aviation Authority (CAA) has 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".
Patricia Cambridge said her daughter was asked to pay again for her half-board holiday.
She told the BBC: "She was told that they would have to pay £438 for the self-catering holiday, plus 10 euros (£8) a day if they wanted half-board. They'd actually booked a half-board holiday and paid Sun4U for that holiday."
A CAA 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."
The CAA said it would ensure that Atol-protected customers were able to fly home as planned. They should therefore arrive at the airport in time to check in for their flights home as normal. Customers with forward bookings for Sun4U air package holidays should not travel, but claim a refund.
Those who booked through Sun4U with other tour operators should contact their operator to check if their holiday can still go ahead. Anyone who booked a Sun4U holiday where payment for their flights went directly to the airline are not Atol-protected and customers who booked accommodation only with Sun4U are also not covered by Atol.