The aviation regulator has warned airlines could face legal action if they mishandle cancelled flight refunds.
The Civil Aviation Authority (CAA) said it was reviewing how airlines were dealing with customer refunds during the coronavirus pandemic
By law, plane operators must refund customers within seven days if their flight is cancelled.
But with most fleets grounded, airlines are struggling to cope with all the requests for refunds and are delaying paying out or directing customers to vouchers.
A statement from the CAA said: "Under the law, consumers are entitled to receive a refund for their cancelled flights, despite the challenges the industry is currently facing.
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"We support airlines offering consumers vouchers and rebooking alternatives where it makes sense for the consumer. But it is important that consumers are given a clear option to request a cash refund without unnecessary barriers."
The regulator said it did not expect airlines to "systematically" deny consumers their right to a refund and they should provide the monies "as soon as practically possible."
Last month, consumer group Which? said it had received thousands of complaints from people struggling to secure a refund for their cancelled travel.
The Competition and Markets Authority (CMA) also received a rise in customer complaints over cancellations and refunds and highlighted concerns over firms refusing refunds or pressuring consumers to accept vouchers. The CMA said companies failing to give refunds could also face court action from them.
But Ryanair boss Michael O'Leary has said it will take up to half a year to refund passengers for coronavirus- related flight cancellations.
Meanwhile airlines are still reeling from a government announcement to introduce a two week quarantine to anyone arriving into the UK from any country apart from the Republic of Ireland.