Colin Blakely and Margaret Whiting were married in real life.
Ex-police officer John Gosling, technical adviser on this movie, was also the author of Ghost Squad (1961), made into a TV series a few years earlier.
When the informer jauntily tells the policeman, "I've never had it so good," he is alluding to the slogan "You've never had it so good," which helped win the Conservatives the general election in 1959.
This British film is one of the first to include a new kind of swear word. It occurs at the end when Derren Nesbitt looks in shock out a window.
The original novel on which this is based was called "Death Of A Snout" - "snout" being underworld slang for a police informer. This title was at first retained for the film when it was first announced and then became simply "The Snout"; but it was felt that this would baffle rather than attract audiences, and so, at a late stage of post-production, became "The Informers" - the title was pluralized so as not to make people think it was a remake of John Ford's famous film of 1935.