Customers are waiting for more than £1bn in refunds due to package holidays being cancelled because of coronavirus.
The estimate from Which? was based on results from a survey of more than 7,500 people who had a package holiday cancelled as a result of the pandemic, as well as population figures.
Around 9.4 million people have had a package holiday cancelled since the UK's first lockdown in March and, of those who requested a refund, 21% were still waiting at the beginning of October.
More than four out of 10 people told Which? they had waited longer than a month for their refund.
The average cost of a cancelled holiday was £1,784, the research found.
Holiday companies are required to refund money within 14 days after a trip is cancelled.
However, the number of cancellations due to the coronavirus pandemic has left many of them overwhelmed.
The situation is set to get worse after new rules are introduced across England later this week, banning most international travel.
Rory Boland, editor of Which? Travel, said: "Without meaningful intervention from the government and the regulators in this space, many people will struggle to get their money back.
"The CMA must take firm action against any operators that are continuing to drag their feet on refunding holidaymakers and the government must urgently set out how it will support travel companies in fulfilling their legal obligations to passengers."
During the summer, the Competition and Markets Authority launched an investigation into package travel companies' handling of cancellations and refunds.
