Via ProofreadNOW, a quick study of two commonly confused words as conducted by Garner’s Modern American Usage, the standard for law-firm style. Garner writes:
Flammable, inflammable. The first is now accepted as standard in American English and British English alike. Though examples of its use date back to 1813, in recent years it has become widespread as a substitute for inflammable, in which some people mistook the prefix in- to be negative rather than intensive. Traditionally, the forms were inflammable and noninflammable; today they are flammable and nonflammable. By the mid-20th century, purists had lost the fight to retain the older forms.
Even staunch descriptivists endorsed the prescriptive shift from inflammable to flammable–e.g.: “A word is bad if it is ambiguous to such a degree that it leads to misunderstanding. For me, the perfect example of such a word is flammable, if it is applied to substances. As most dictionaries now recognize, inflammable can be confused with non-combustible, and so lead to accidents.” Archibald A. Hill, “Bad Words, Good Words, Misused Words,” in Studies in English Linguistics for Randolph Quirk 250, 252 (1983).