Travel company Sun4U has collapsed, leaving approximately 1,200 customers stuck abroad, most of them in Spain.
It said it had suffered significant losses due to the volcanic ash and the failure of tour operator Goldtrail.
Sun4U apologised to customers and told them to contact claim handlers for Abta – The Travel Association.
But Abta said most had been booked by Sun4U with another non-Abta hotel or agent, so were not covered by Air Travel Operators’ Licensing protection.
It said those bookings should be honoured regardless of whether the companies had received payment from Sun4U, but in the event they were not, customers should claim on their credit card or Visa debit where possible.
Abta added that those with Atol-protected packages should be able to continue with their holidays as planned.
Nigel Barnes and his family paid £604 to Sun4U for an all-inclusive hotel on the Costa Brava, but they and other guests have now been asked to pay again for the whole week or leave.
“They wanted 1,124 euros [£924] for our stay,” he told the BBC.
“People were saying, ‘Well, we just can’t afford that. We paid all-inclusive so we didn’t have to spend any money while we’re away.'”
Mr Barnes went to a local travel agent to find alternative accommodation and was told that the cheapest option for his remaining two nights was back in his original hotel.
He fears he may not be eligible for a refund because when he booked he paid Sun4U for his accommodation, but was told by the company to pay Ryanair directly for his flights.
“[The Atol website] was saying if you’d booked as a complete thing it was Atol-protected, if you hadn’t it wasn’t,” he said. “So, I don’t know where we stand money-wise.”
The BBC’s Sarah Rainsford said another group of 20 Sun4U holidaymakers in the resort of Lloret de Mar had been notified over the tannoy as they were sunbathing that the all-inclusive deal they had paid for was no longer valid.
One woman said her family of five was presented with a bill of 2,700 euros (£2,220) – more than the original cost of their holiday – and told they must pay it before they would be allowed any more food or drink.
Our correspondent said hoteliers were also suffering, and one in particular had 100 rooms currently booked through Sun4U, but had not been paid for any of them.
Rebecca Smyth and three friends booked their holiday to Benidorm on 10 August.
She told the BBC: “We found out today that the firm went into liquidation literally 24 hours after we had booked.
“I do not know where I stand with money now or whether or not we are going on holiday. I find it absolutely disgusting.
“Surely they knew the day before that they were going to go bust.”
In a statement, Sun4U said Goldtrail had been one of its major tour operators and its collapse last month had been damaging.
“The directors of Sun4U sincerely apologise for the disruption caused by the demise of the company and of course the loss of jobs including many staff who have been with the company from the outset,” it added.
Keith Cornforth, 39, from Wolverhampton, was hit by the collapse of both companies and has lost £2,347.
He said: “I booked a complete holiday in February with Sun4U to Turkey, but then when Goldtrail got into trouble I got an e-mail saying my flights were with them and that I’d need to find alternative ones.
“I called Sun4U on Tuesday and they offered me new flights for an extra £1,300. I didn’t book because I didn’t have my passport on me, but now they have gone down. If I booked those flights I’d have lost even more money.”
Sun4Udirect.com advertised a “dynamic packaging” service offering “much greater choice and greater flexibility” and a team of staff with “hundreds of years of experience” between them.
It specialises in holidays on the Spanish coast and Majorca.
The HTOP hotel chain in Spain, which has 14 hotels in and around Calella de Mar and Playa de Aro on the Costa Brava, says passengers are able to still go on holiday if they wish, but they will have to pay again.
For passengers already in a hotel, their rooms are covered, but if they were all inclusive, that is now stopped and they will have to pay for food and drinks themselves.
Anyone affected by the collapse is advised to call the Civil Aviation Authority on 08445 717260.
People on an Atol-protected Sun4U package who are asked to pay again for accommodation will be able to claim a refund from the CAA or from their credit card company if they paid by credit card.
Customers due to leave on a package trip in the coming weeks are advised not to travel and to seek a refund.
Sun4U also sold accommodation separately and this is not covered by Atol. Customers in this position who paid by credit card or Visa debit must seek a refund from the card provider.
Anyone who purchased a flight only through Sun4U should be unaffected if the payment was made direct to the airline. If in doubt, contact the airline.