Britain on Friday warned Russian President Vladimir Putin and Chinese President Xi Jinping that the West would stand together to fight for democracy against dictatorships which it said were more emboldened than at any time since the Cold War.

Western leaders said the 21st Century will be defined by a struggle between democracies and rivals such as China and Russia who they said are challenging the post-Cold War consensus militarily, technologically and economically.

Speaking in Australia, British Foreign Secretary...