Many of the flights were not cancelled, even though passengers could not reasonably - or in some cases legally - travel.
Some 2.3 million people have not received money back for flights that they could not take during the coronavirus
pandemic, according to a new estimate.
The figure comes from consumer group Which? and is based on a survey of more than 2,000 people in the UK during February.
The flights were not cancelled, even though passengers could not reasonably - or in some cases legally - travel to their destination, due to restrictions in the UK or foreign countries.
Survey participants were asked if they had booked a flight that had gone ahead since March last year that they could not take, and if they had received a refund.
Which? said that if an airline did not cancel a flight, then customers were not legally entitled to a refund or guaranteed a successful claim through their travel insurance or bank.
But many of these passengers were prevented from travelling due to national lockdowns, the government advising against non-essential travel, or restrictions preventing them entering their destination country.
Of those who told Which? they did not get refunds, 49% claimed they could not travel because of national or regional lockdown restrictions instructing them to stay at home.
Most passengers in these circumstances would have been given the choice of rebooking - often meaning paying a significant difference in fare - or losing their money, Which? said.
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."
The survey has been shared with the Competitions and Markets Authority for its investigation into whether airlines have breached consumers' legal rights by failing to offer cash refunds for flights they could not lawfully take.
Which? said anyone considering booking flights for this summer should wait until the rules and restrictions around international travel are clearer.
It suggested booking a package holiday rather than a flight-only booking for stronger passenger protections, and only booking with a trusted provider with a generous and flexible booking policy.