Guidance issued by the European Commission in May recommends airlines automatically refund unused vouchers 14 days after they expire.

According to Which?, though, British Airways and easyJet are insisting that once issued, vouchers cannot be exchanged for cash – meaning passengers could be left out of pocket if they do not use them before they expire.

EU airlines, or airlines operating from EU airports, are obliged under EU Regulation 261 to provide cash refunds for cancelled flights within seven days of cancellation.

However, many airlines opted to flout the law and issue vouchers instead when air travel largely ground to a halt in March and April.

Meanwhile, Which? said many passengers subsequently struggled to get hold of their airline to request a cash refund, or complained of what they felt were deliberately obstructive or misleading practices.

Airlines have since reduced their networks significantly or actively closed bases owing to the pandemic, rendering vouchers useless to some passengers within their limited timespan.

Which? Travel editor Rory Boland said: “As we head into a winter that is bound to bring more flight cancellations, it’s extremely concerning to see the UK’s biggest airlines disregarding European guidance and letting their passengers down when it comes to their refund rights.

“BA and easyJet must immediately make it clear that passengers will not face losing their money if they are unable to use a voucher, while all airlines should be offering cash refunds to passengers prevented from travelling by lockdown laws.”

Boland added some of the UK’s major airlines had “acted shamefully” and “without fear of consequences”, and urged the government to review the CAA’s powers to police airlines refund practices.

An easyJet spokesperson said: “For passengers who have chosen a voucher as compensation for their cancelled flight, we would only reverse this and offer a refund in exceptional circumstances as a gesture of goodwill if the customer’s circumstances justify it.”

A BA spokesperson said: “We do not auto-issue vouchers, they can only be issued when a customer has requested them by filling out the form. Our website is clear that when filling out the form, it is to apply for vouchers.

“Customers are always entitled to a cash refund if their flight has been cancelled, and must call us to do this, which is clearly displayed on our website. Customers have up to a year after their flight was due to operate to get in touch with us for a cash refund – and we have processed over 2.1 million cash refunds to date.

“If a customer uses a voucher to make a new booking which is then subsequently cancelled by us, they would be entitled to choose either a cash refund (if the original booking was paid for in cash), or a voucher in a similar way as before.”