The -ise ending is actually newer than the -ize ending. In Britain and other countries, it became popular after 1875, when it began appearing in news articles. However, the -ise ending didn’t catch on in the United States or in British science periodicals and professional journals. For that reason, you will see both verb endings in British literature.
Oxford University Press, a British publisher, prefer to use the -ize ending for words that derive from the Greek suffix -izo. Doing so reflects the origins of verbs and nouns, such as realization, organization, and privatization. An -ise ending could erroneously suggest that the verbs derive from the French verbs réaliser, organiser, or privatiser. That’s not to say that Oxford style always condemns -ise endings. To illustrate, the -ise of televise doesn’t have a Greek origin, so that -ise ending is A-okay.
