The consumer group said holidays to some British destinations currently cost hundreds of pounds more than trips to equivalent European destinations, even when airfare is factored in.
The cost of a seven-night break this August to a "highly rated hotel" in Italy was reportedly around a quarter of the price of a comparable holiday in the Lake District.
Seven-nights accommodation in Lake Garda cost £631, while a week-long stay for two people in Lake Windermere would cost £2,381.
Data from AirDNA also showed that UK self-catered accommodation in August 2021 costs 41% more on average than it did in 2019. This works out at roughly the equivalent of an extra £300 per week.
Rory Boland, Which? travel editor, said even though domestic holidays have "always been expensive", the situation has become "far worse" during the pandemic.
"The reasons for these higher prices are complex, but it is hard to avoid the conclusion that some unscrupulous accommodation providers are charging over the odds," he added.
The findings came as Irish online travel agent Click&Go discovered for €683pp, holidaymakers could book seven-nights in the VIK San Antonio in Lanzarote, including bed and breakfast, dinner each night and return Aer Lingus flights from Dublin.
A similar seven-night holiday in a well known hotel in the south of Ireland would cost €859, which is €176 more expensive per person and does not include transport or dinners.
Paul Hackett, chief executive of Click&Go, said the price point of many domestic breaks are "simply too high" and consumers would get better value for money if they were to holiday overseas.
"As we have demonstrated with this comparison, you get more value for less money on a holiday to Lanzarote than you would if you stayed in Ireland," he added.
