The Oxford English Dictionary has always stated that "ize" is the primary spelling of words such as "realize" and that "ise" is the alternate spelling. Not the other way around. It is a common (mistaken) belief in the UK that "ize" is the "American" spelling.

e.g.

http://www.askoxford.com/concise_oed/orexxalize?view=uk
http://www.askoxford.com/concise_oed/orexxcognize?view=uk

http://www.chambersharrap.co.uk/chambers/features/chref/chref.py/main?title=21st&query=realise
http://www.chambersharrap.co.uk/chambers/features/chref/chref.py/main?query=recognise&title=21st

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