The wet summer put a damper on visitor numbers to top tourist attractions with some sites having their worst year ever, according to latest figures.
With the Olympics also affecting tourism, some London attractions hosted 60% fewer visitors during the Games, the Association of Leading Visitor Attractions (Alva) said.
Visits to gardens and heritage spots in London were particularly badly hit this summer by the "appalling weather", with the amount spent during visits also falling.
Among the attractions that saw a dip in numbers this summer were London Zoo, the Tower of London and Kew Gardens.
Overall, Alva member sites in London saw an average decrease in visitor numbers in May-August compared with the same period last year.
Garden and leisure sites in London received 21.3% fewer visitors this summer, while the capital's heritage and cathedral sector was down 20.3% and the museum and galleries sector fell 13.1%.
Visitor numbers to Alva member sites also fell in the rest of England and in Scotland, but not as markedly as in London.
Alva chief executive Bernard Donoghue said: "These figures from our 43 members, who manage nearly 2,000 tourist sites and welcome over 100 million domestic and overseas visitors each year, are definitely sobering reading and show that the summer of 2012 has been a difficult time financially for our most popular and best-loved visitor attractions.
"It is our belief that for gardens and outdoor attractions across the UK, the appalling weather during much of the year has led to one of the worst trading periods since 2001 - the year of a foot-and-mouth disease outbreak.
"For London attractions, the Olympic period was one of their worst trading periods in living memory and for visitor attractions, the summer is their equivalent of retailers' Christmas. Once lost, the business can't be won back.
"Alva has always taken the long-term view that the economic benefits for tourism of hosting the Olympic and Paralympic Games would be long term rather than short term.
"We are working with the local and national tourist boards and others to turn the millions of Olympic TV viewers who loved how Britain looked into visitors who will come here in the next months and years."
Data released by MeteoGroup, the weather division of the Press Association, at the end of August, showed that 14.25in (362mm) of rain has fallen in June, July and August, making it the wettest summer since 1912.