Both. ;-) Click here: British, Canadian and American Spelling
In other words recognise is a British English spelling of recognize.
I usually find that the better dictionaries list my preferences first as well. :-DGood English dictionaries (including the Oxford English Dictionary and Chambers) show 'recognized' as the main option with -ise as an alternative. This is certainly my preference. To suggest that ize is a US invention is incorrect. The 'z' was good enough for William Shakespeare (and I doubt if he was influenced by anyone over the Atlantic in the late 16th century!) so it's fine with me.
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