Holidaymakers are being urged to use unspent vouchers worth £30 million before they lose financial protection next week.
Consumers who fail to act by September 30 are putting their money at risk, the Civil Aviation Authority (CAA) said.
Refund credit notes (RCNs) were given to customers by travel firms for package holidays cancelled due to the coronavirus pandemic.
Vouchers worth a total of £30 million have not been used, according to the CAA.
RCNs can be put towards the cost of a new holiday or exchanged for cash.
Any issued between March 10 2020 and December 19 2021 are covered by the Atol scheme, which means holders will not lose out if the company they booked with goes bust.
But that protection expires at the end of the month.
Michael Budge, head of Atol, which is run by the CAA, said: “We are again urging people to either use their refund credit note to book another Atol-protected trip or ask their travel company for a refund.
“This will affect customers of some of our biggest travel firms, so our message is simple – this is the last chance to make sure you continue to be financially protected by the Atol scheme.
“Even if your refund credit note is valid after the end of September, it won’t enjoy Atol protection.
“It is better to be safe than sorry, and we are urging people still holding one of these refund credit notes to take action now.”
Consumers were legally entitled to cash refunds within 14 days for package holidays cancelled due to the virus crisis.
But many travel companies offered RCNs as an alternative to help their cash flow during the widespread disruption.