Holidaymakers were today having to pay for their overseas accommodation for a second time following the collapse of tour operator Sun4U.
And 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.
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."
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 CAA spokeswoman said today: "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.
Kellie Bugby was due to travel to Majorca in two weeks' time with Sun4U along with 36 relatives.
She told the BBC: "We've tried the website, we've tried calling. We're still a bit lost about whether we are or aren't going. It's something we've been planning for a whole year. It's over £12,000 of money."
Ms Bugby, from Leicester, said she checked the Sun4U website three days ago and the company appeared to be still selling holidays.
She added: "They must have known that the ones they were still selling weren't going to go through. It's so unfair."