The UN's top court on Wednesday ruled climate change an "existential problem of planetary proportions" and said countries had a legal duty to act or face consequences.

One of the world’s most vulnerable countries to the fallout from climate change has won a landmark legal battle, giving countries new ammunition to pursue some of the planet’s biggest emitters.

The United Nations’ top court in a landmark advisory opinion says countries could be in violation of international law if they fail to take measures to protect the planet from climate change, and nations harmed by its effects could be entitled to reparations.