What's the name of the film where Japanese kids get put on an island to kill each other?

I watched a really good film ages ago set in the future in Japan about where the population is too big so they take handfuls of children and leave them on an island to be killled and kill each other and only one can survive at the end. It's really gory but good. I can't remember the name does ayone have any suggestions? >.< thank you =)