Agents who have dynamically packaged flights with failed airline Ajet face having to decide whether to rebook with other carriers at their own expense or tell clients they are on their own.
The carrier, part of the Libra Holidays Group, was grounded last week by Cypriot authorities and its assets frozen in a dispute about unpaid tax.
Libra declined to comment, but it is estimated between 8,000 and 12,000 UK customers could be involved.
The carrier is advising customers who booked through an agent to deal with the agency. Those who booked directly must apply for a refund in writing, which it will review when it is in a position to do so.
Tour operators will make alternative arrangements for customers on package holidays. Sunvil Holidays managing director Noel Josephides said: “Some agents will help their clients and some won’t.”