Flight-only specialist Fone N Fly has ceased trading.
The company, which operated the website fonenfly.co.uk, was based in Ealing, London, and offered flights worldwide.
It failed on August 18 with 132 customers abroad and 226 forward bookings, accounting for 450 clients, according to the Civil Aviation Authority.
Airlines should honour tickets for those clients currently overseas, said a CAA spokesman. But the CAA has advised clients overseas to contact their airline before travel to check their return ticket is still valid.
If it has been cancelled by the airline, customers are advised to book a reasonably-priced ticket and apply for a refund on their return via their airline, and if unsuccessful, contact the CAA.
“Most people will probably be able to return as normal but if their ticket has been cancelled they should rebook at a reasonable cost and challenge the decision to cancel their booking on their return to the UK,” said a CAA spokesman.
Clients still due to travel should contact their airline to see if their ticket remains valid. They can claim a refund via the CAA.
Clients who booked by credit card should contact their card issuers, which will process refunds.