Millions of people are out of pocket on flights they were unable to board due to coronavirus restrictions, according to consumer group Which?.
Holidaymakers prevented from flying by travel restrictions or lockdowns may not be entitled to a refund.
More than two million people have not got their money back, Which? said.
Travel restrictions remain in place in the UK, with travellers arriving from "high-risk" countries having to stay in quarantine hotels.
Most of the people who were unable to take flights were hit by circumstances which meant they reasonably, or in some cases, legally, could not travel to their destination, Which? said.
It is warning people to be wary of booking flights for this summer and to wait until the situation around international travel becomes clearer.
The consumer group said in the longer-term booking a package holiday rather than a flight-only booking should give stronger passenger protections. People should only book with a trusted provider with a generous and flexible booking policy, it said.
No flight refund
Travel restrictions introduced at the Spanish border on Christmas Eve meant the Howe family couldn't take their trip to Lanzarote.
They were able to cancel their accommodation free of charge, but it was a different story with their flights.
"We asked for a refund, but was told as the flight was still operating, we weren't entitled," Rebecca Howe told the BBC.
It has left the family £980 out of pocket, and they're far from alone in losing cash paid for flights.
Prevented from travelling
Hopeful holidaymakers have been prevented from travelling because of local or national lockdowns and restrictions preventing entry at their destination, or the Foreign Commonwealth and Development Office advising against non-essential travel.
But in cases where the airline did not cancel the flight, customers may not be legally entitled to a refund or guaranteed a successful claim through their travel insurance or bank.
Which? said passengers in these circumstances would often have only been given the choice of rebooking their flight or losing their money.
Rebecca Howe said she was offered the choice to rebook her Ryanair flights for free up until the end of March, but that was no good.
"For one thing all the Ryanair flights to Lanzarote between Christmas and the end of March were less than half the price as we paid a premium to travel over New Year.
"If we'd accepted a cheaper flight, we would have lost the difference."
But with the only practical date available for the family to go was February half-term, even that was no good.
"Someone at my work had already it booked as annual leave so I couldn't take the time off as we are only allowed one person out."
They're now trying to reclaim the money via a chargeback request through their bank, "But there's no guarantee it'll be successful so at the moment we're accepting we have lost the money."
Donna Smullen and her husband from Cheshire had booked flights to Spain with Ryanair in September 2019, long before the start of the pandemic.
They were due to travel in August 2020, but at the end of July, the FCDO warned against non-essential travel to Spain. That warning invalidates most travel insurance.
A two-week quarantine was also introduced for flights returning from mainland Spain. Due to their jobs, quarantining was not an option for the couple.
"We were told by Ryanair we could change the date of our flights for a fee of €30 to €90 (£26 to £78) per person per flight, as long as we travelled before the end of 2020," she said.
That meant rebooking would have been more expensive than the flights themselves, so they took the hit and were left more than £500 out of pocket.
Ryanair said that passengers who book non-refundable flights are not entitled to refunds if they choose not to travel on flights which have operated.
However, passengers can avoid being out of pocket by "availing of Ryanair's change facility, even for bookings which were made prior to any Covid-19 flight restrictions being introduced."
Which? has shared its findings with the CMA to aid an investigation into whether airlines have breached consumers' legal rights by failing to offer cash refunds for flights they could not lawfully take.
Rory Boland, editor of Which? Travel, said: "With non-essential travel currently illegal, airlines must play their part in protecting public health by ensuring no-one is left out of pocket for abiding by the law and not travelling.
"All airlines should allow passengers the option to cancel for a full refund, as well as fee-free rebooking options, while these restrictions remain in place."