China's territorial claims in the South China Sea, in defiance of a 2016 international arbitration ruling, is the "gravest external threat since World War II" to the Philippines, the acting head of the nation's Supreme Court has said.

At a forum in Manila on Saturday, Senior Associate Justice Antonio Carpio encouraged Filipinos to ask the entire world to help convince the Chinese to abandon the belief China has a historic claim to almost the entire South China Sea.

"In China today, all the generals, admirals, professors, bureaucrats, diplomats, businessmen, they all grew up being taught from Grade One to college that they own the South China Sea since 2,000 years ago. It is in their DNA. They really believe it, honestly, sincerely. But it's totally false," Carpio said in a lecture entitled "Defending Philippine Sovereign Rights in the West Philippine Sea."