The Film Distributors Association says 21.8 million cinema visits were made in July, the equivalent of one in three of the population.
The previous highest monthly total in recent years was 19.0 million in February 2002.
So far this summer, five releases - Harry Potter and the Order of the Phoenix, Shrek the Third, Pirates of the Caribbean: At World's End, The Simpsons Movie and Spider-Man 3 have broken into the top 30 of the UK's all-time records box-office successes.
Smaller titles such as SherryBaby, Tell No One, La Vie En Rose and Partner have also made strong showings.
FDA Chief Executive, Mark Batey, said: 'It's really exciting that audiences right across the generations have enjoyed an emotional work-out in the cinema this summer. We're looking forward to a wonderful autumn and a record-breaking year.'