China escalated its criticism of planned maritime boundary talks between Japan and the Philippines, issuing a legal opinion that says the proposed delimitation in waters east of Taiwan “constitutes a severe violation of international law” and an “internationally wrongful act.” The document, released by the China Institute for Marine Affairs, a think tank under the Ministry of Natural Resources, argues that the areas under discussion substantially overlap with China’s exclusive economic zone and continental shelf claims under the United Nations Convention on the Law of the Sea, or UNCLOS.

The opinion targets negotiations announced in late May during Philippine President Ferdinand Marcos Jr.’s visit to Japan, when Tokyo and Manila said they would begin formal maritime delimitation talks. According to the Beijing-based institute, the decision to move ahead without consulting China disregards what it describes as the “specific geographical circumstances” of the region and violates principles of sovereign equality, good faith, cooperation and self-restraint in international law. The paper contends that by negotiating bilaterally, Japan and the Philippines are attempting to bypass a direct stakeholder.

China’s Foreign Ministry had already condemned the initiative shortly after it was unveiled, with spokeswoman Mao Ning saying the move “severely violates” China’s maritime rights and interests and UNCLOS. Beijing has lodged formal diplomatic protests with both governments, and relevant Chinese authorities, including the China Coast Guard, have stepped up patrols in waters east of Taiwan in recent days. At a news conference, Zhu Fenglian, spokeswoman for the State Council Taiwan Affairs Office, described the proposed talks as “completely illegal, null and void,” asserting that the coast guard activities in the area are a lawful exercise of jurisdiction.

The legal opinion goes beyond procedural objections, warning Tokyo and Manila not to “challenge the one-China principle under the pretext of ‘delimitation’.” It calls on the two countries to halt the talks and instead enter into consultations with China, and urges third states not to assist what it characterizes as an internationally wrongful act. While Japan and the Philippines have framed their move as a step toward clarifying overlapping maritime entitlements, Beijing’s latest intervention underscores how efforts to draw formal sea boundaries in the Western Pacific are increasingly intersecting with broader disputes over sovereignty, jurisdiction and the legal interpretation of UNCLOS.