Philippine President Benigno Aquino's office said Monday it was renaming the South China Sea as the "West Philippine Sea", as tensions with Beijing mount over the disputed area.
The Philippines and China, along with Brunei, Malaysia, Taiwan and Vietnam have competing claims to areas of the South China Sea, most importantly the Spratly Islands that are believed to sit on vast oil and gas resources.
Presidential spokesman Edwin Lacierda said the foreign affairs and defence departments recently began using the term West Philippine Sea instead of the South China Sea, and the president's office had decided to follow suit.
"It is incumbent on us to take the cue from them and to refer to South China Sea as West Philippine Sea," he said.
Lacierda pointed out other countries had similar practices.
"All the other nations call the South China Sea based on how they perceive it. Vietnam calls it East Sea so it is but natural for us to call it West Philippine Sea," he said.
The area of water to the east of the island nation is already known as the Philippine Sea, so that the new name threatens possible confusion over the western part of that sea.
Philippine foreign department spokesman Ed Malaya said his agency had first publicly referred to the "West Philippine Sea" on June 1, but even before that had used the term in communications with China.
In recent weeks, the Philippines has publicly accused Chinese forces of being behind seven incidents or confrontations with Filipinos in and around the Spratlys.
Chinese ambassador Liu Jianchao said the reported incidents were mere "rumours" or exaggeration, but insisted on China's sovereignty over the Spratlys.
As regional tensions mount, Taiwan announced over the weekend it was planning to deploy missile boats to the South China Sea and tanks on some of the islands.
Vietnam held live-fire naval drills in the South China Sea on Monday.
Lacierda said Manila would increase its defences in the area "to enable the effective patrol and protection of our national territory".