Merriam-Webster says this:

“When cooking over a gas stove, avoid wearing loose, (flammable/inflammable) clothing that could catch fire easily.” Which word is correct: flammable or inflammable?

It’s a trick question. Both flammable and inflammable are correct, as they both mean “capable of being easily ignited and of burning quickly.” This makes no sense to the Modern English speaker. In English, we think of in- as a prefix that means “not”: inactive means “not active,” inconclusive means “not conclusive,” inconsiderate means “not considerate.” Therefore, inflammable should mean “not flammable.”

[…]

What do you do? If you want to keep things crystal clear, choose flammable when you are referring to something that catches fire and burns easily, and use the relatively recent nonflammable when referring to something that doesn’t catch fire and burn easily. Inflammable is just likely to enflame confusion.

Personally I use “inflammable”, but if I were writing warning posters I would have to reconsider that.