Microsoft, in its infinitesimal wisdom, thinks that most words ending in -ize (or -ized, -izes, -izing or -ization) in US English end in -ise (etc.) in British English. Well, that style does seem to be gaining ground – and no doubt Microsoft’s insistence on it being the case is consolidating that.
When I was at school in the 1950s and 1960s, it was definitely -ize in British English*. I remember one English teacher telling us that -ise was an Americanism that was creeping in, and I’d always believed it. But I was recently taken to task for using -ize in my book, The Reminiscences of Penny Lane, so I decided to research the question.
Well, my teacher was right that -ise was creeping into British English, but he was wrong about it being an Americanism. I don’t know where it came from, but it wasn’t America.
Our bookshelves at home contain an extensive collection of books published on both sides of the Atlantic, dating from the late 19th century to the early 21st, including numerous dictionaries and “Guides to Good English”, and a quick check reveals that it’s always been -ize in US English, and that British English was the same until sometime around the mid-1960s, when -ise began to creep in – in the UK only, as far as I can tell†. Before that, neither British nor American dictionaries even offered the option of -ise, and books didn’t use it. -ise didn't really get much of a hold for quite a while. The Oxford Dictionary for Writers and Editors still said “organize not organise” in 1981. Hart’s Rules (also OUP) said, in 1978, “use -ize in preference to -ise as a verbal ending in cases where both spellings are in use.”
By 1986 Gowers, in The Complete Plain Words (HMSO), was saying,
On the question of whether verbs like organise and nouns like organisation should be spelt with an s or a z the authorities differ. There are some verbs (e.g. advertise, comprise, despise, advise, exercise and surmise) which are never spelt with a z. There are others (such as organize) for which the spelling with a z is the only American form and is also a very common British one. This being so, the British writer has the advantage over the American that we may, if we wish, use an s all the time, for that will never be wrong, whereas a z sometimes will be. But do not condemn those who use a z in its right place.
It still seems to be a matter of house style which individual publishers use. Some don’t even bother to be consistent, some take the trouble to be consistent within each publication but differ from one publication to another, others sometimes aren’t even consistent within individual publications.
*This is about the ones that are spelt -ize in US English. There are many words that are spelt -ise in both US and British English, of course. The dictionary is your friend!
†Update: my brother has found a number of earlier examples of -ise in English publications, and I’ve recently acquired a set of atlases published in the 1950s that have -ise as well. But none of my older dictionaries even give it as an option!