Holiday companies are required to refund the money within 14 days but the number of cancellations has left many of them overwhelmed.
Customers are waiting for more than £1bn in refunds due to package holidays being canceled 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 canceled as a result of the pandemic, as well as population figures.
Around 9.4 million people have had a package holiday canceled 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 canceled holiday was £1,784, the research found.
Holiday companies are required to refund the money within 14 days after a trip is canceled.
However, the number of cancellations due to the coronavirus pandemic has left many of them overwhelmed.