The aviation watchdog has warned airlines that they are legally required to provide refunds to customers who had their flights cancelled because of the coronavirus.
By law, plane operators must refund customers within seven days if their flight is cancelled.
But with fewer than 10% of UK flights taking off, airlines are struggling to deal with all the requests for refunds.
The Civil Aviation Authority (CAA) said it could take action against airlines.
"We are reviewing how airlines are handling refunds during the coronavirus pandemic, and will consider if any action should be taken to ensure that consumer rights are protected," the regulator said in a statement.
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Last month, consumer group Which? said it had received thousands of complaints from people struggling to secure a refund for their cancelled travel. Instead, airlines were offering customers vouchers to be used when lockdown are lifted.