On 29 October 2018, the Malacañang, through Presidential Spokesman Salvador Panelo, confirmed that the Philippines is set to sign three deals with China for the joint exploration of the West Philippine Sea (WPS).[1] This was despite a landmark victory in the South China Sea arbitration,[2] where an arbitral tribunal constituted under the United Nations Convention on the Law of the Sea (UNCLOS) recognized that the Philippines has a full 200-nautical mile (NM) exclusive economic zone (EEZ) in the WPS, and consequently, the exclusive right to explore and exploit the resources within the area.[3]


The plans for joint exploration have been heavily criticized. Supreme Court Justice Antonio Carpio expressed his concerns about the move’s constitutionality,[4] while former solicitor general Florin Hilbay considered it “blatantly unconstitutional.”[5] Both stressed that the Constitution imposes a duty upon the State to reserve the use of the nation’s marine wealth exclusively to Filipino citizens.[6]


Perhaps even more alarming is the legal possibility that the Duterte administration’s “defeatist, self-flagellating stance”[7] on the WPS issue may constitute estoppel – a general principle of international law[8] that “bars a party from going back on its previous representations when those representations have induced reliance or some detriment on the part of others.”[9] This essay examines how the principle may be invoked against the Philippines, and what that means to its maritime rights over the WPS.


Estoppel in International Jurisprudence

Estoppel, when properly invoked, may establish a loss of right.[10] The principle proved to be relevant in a number of international judgments, most notably in Temple of Preah Vihear. In ruling that Cambodia had sovereignty over the Temple,[11] the International Court of Justice held:[12]


Even if there were any doubt as to Siam’s acceptance of the map in 1908, and hence of the frontier indicated thereon, the Court would consider, in the light of the subsequent course of events, that Thailand is now precluded by her conduct from asserting that she did not accept it. She has, for fifty years, enjoyed such benefits as the Treaty of 1904 conferred on her, if only the benefit of a stable frontier. France, and through her Cambodia, relied on Thailand’s acceptance of the map. (emphasis supplied)


Requisites of Estoppel

In the Chagos Marine Protected Area arbitration, the arbitral tribunal constituted under the UNCLOS consolidated the requisites for the invocation of estoppel as follows:[13]


(a) a State has made clear and consistent representations, by word, conduct, or silence;

(b) such representations were made through an agent authorized to speak for the State with respect to the matter in question;

(c) the State invoking estoppel was induced by such representations to act to its detriment, to suffer a prejudice, or to convey a benefit upon the representing State; and

(d) such reliance was legitimate, as the representation was one on which the State was entitled to rely.


The next parts assess the application of these requisites in the WPS issue.


  1. Clear and Consistent Representations

In discussions regarding the possible legal effects of the Duterte administration’s conduct regarding the WPS, the concepts of acquiescence[14] and implied waiver[15] have generally been used. While it is unclear whether there exists a distinction between these two,[16] the same cannot be said when both are compared with estoppel. Judge Gerald Fitzmaurice, in his separate opinion in Temple,[17] observes:


[Estoppel] is quite distinct theoretically from the notion of acquiescence. But acquiescence can operate as a preclusion or estoppel in certain cases, for instance where silence, on an occasion where there was a duty or need to speak or act, implies agreement, or a waiver of rights, and can be regarded as a representation to that effect. (emphasis supplied)


For the purpose of this article, acquiescence and implied waiver are used interchangeably as a form of representation in examining the application of the first requisite.


Acquiescence is “the inaction of a State which is faced with a situation constituting a threat to or infringement of its rights,”[18] and may take the form of “silence or absence of protest in circumstances which generally call for a positive reaction signifying an objection.”[19] Since 2016, President Rodrigo Duterte has drawn flak for his supposed inaction on China’s activities in the WPS, particularly its reclamation of the area.[20] It is feared that such conduct, coupled with a declaration that China is a “co-owner”[21] of the WPS, may amount to an implied waiver of the Philippines’s maritime rights in favor of China.


The most cogent evidence is the Duterte administration’s decision to enter into joint exploration agreements with China, which officially declared that its concept of joint development consists of the following elements:[22]


  1. The sovereignty of the territories concerned belongs to China.
  2. When conditions are not ripe to bring about a thorough solution to territorial dispute, discussion on the issue of sovereignty may be postponed so that the dispute is set aside. To set aside dispute does not mean giving up sovereignty. It is just to leave the dispute aside for the time being.
  3. The territories under dispute may be developed in a joint way.
  4. The purpose of joint development is to enhance mutual understanding through cooperation and create conditions for the eventual resolution of territorial ownership. (emphasis supplied)


In his book, Justice Carpio warns that to accept this concept of joint development is to make two fatal concessions: (1) that China has sovereignty over areas within the nine-dash line, including the WPS, and (2) that the WPS issue involves a territorial dispute – a subject matter beyond the jurisdiction of the arbitral tribunal in South China Sea[23] – rendering the resulting award null and void.


Still, it is difficult to claim that the twin requirements of clarity and consistency have been met. Even when his conduct shows otherwise, President Duterte, on multiple occasions,[24] has vowed to defend the Philippines against China’s expansive claims. In May 2018, the Philippines reportedly filed a note verbale against China, albeit the details were not disclosed to the public.[25]


However, even in the absence of these twin requirements, estoppel may arguably still exist as long as the State against whom the principle is invoked “behaved in a misleading manner”[26] as to induce the State invoking it to, among others, “convey a benefit upon the representing State.”[27] In other words, notwithstanding President Duterte’s unclear and inconsistent statements, estoppel may still be invoked if it is shown that his conduct legitimately induced China to convey benefits to the Philippines.


  1. Authorized Agents

President Duterte’s statements may call for the application of a distinct but analogous concept known as the principle of unilateral declarations, which was articulated by the International Law Commission as follows:[28]


Declarations publicly made and manifesting the will to be bound may have the effect of creating legal obligations. When the conditions for this are met, the binding character of such declarations is based on good faith; States concerned may then take them into consideration and rely on them; such States are entitled to require that such obligations be respected. (emphasis supplied)


It is widely accepted that “heads of State, heads of Government and ministers for foreign affairs”[29] are vested with the power to make unilateral declarations that bind the State.


While the arbitral tribunal in Chagos applied the principle of estoppel and not that of unilateral declaration, it nonetheless affirmed that the Prime Minister of the United Kingdom, as its head of government, was similarly authorized to make representations that may constitute estoppel.[30] In the same way, since the President is the Philippines’s head of State of the Philippines, President Duterte falls squarely within the meaning of “authorized agents.”


  1. Induced to Convey Benefits

In the Pedra Branca/Pulau Batu Puteh case, the Court declared that for a claim of estoppel to stand, the “party relying on an estoppel must show, among other things, that it has taken distinct acts in reliance on the other party’s statement.”[31]


According to September 2018 documents from the National Economic and Development Authority, Chinese loans are to finance Philippine infrastructure projects amounting to ₱ 743 billion.[32] While the Department of Finance has claimed that the loans were not granted in consideration of the joint exploration agreement,[33] a contrary claim by China may seem more credible based on the surrounding circumstances. Strikingly, it was after President Duterte expressed his eagerness to enter into the agreement in August 2017[34] that the Philippines, in November 2017, was able to secure loans under the “first basket of infrastructure projects” it presented to China for financing.[35]


  1. Legitimate Reliance

In Chagos, the arbitral tribunal declared that the requirement of legitimate reliance is not met if the State invoking estoppel relied “upon an expressly non-binding agreement,” or “upon an expressly revocable commitment.”[36]


In the WPS issue, representations were made in the form of a joint exploration agreement. In this regard, the Constitution allows the President to “enter into agreements with foreign-owned corporations involving either technical or financial assistance for large-scale exploration, development, and utilization of minerals, petroleum, and other mineral oils.”[37] In the case of USAFFE Veterans Association, Inc. v. Treasurer of the Philippines,[38] the Supreme Court held:


[E]xecutive agreements may be entered into with other states and are effective, even without the concurrence of the Senate. … From the point of view of international law, there is no difference between treaties and executive agreements in their binding effect upon states concerned, as long as the negotiating functionaries have remained within their powers.” (emphasis supplied)


If the joint exploration agreement is construed to be a binding and irrevocable one, the same may constitute as legitimate reliance on the part of China.



Justice Carpio, in a speech he delivered on 12 July 2017, remarked that the loss of the WPS is the “gravest external threat to the Philippines since World War II.”[39] In a few days, as pen meets paper, the nation may be crossing the point of no return. While it is easy to surrender to such fate, this author remains to be hopeful. From the People Power Revolution to the more recent restoration of the Commission on Human Rights’s budget, the Filipinos proved that an unwavering fight for justice can succeed.


Margaret Mead, an American cultural anthropologist, could not have said it better: “Never doubt that a small group of thoughtful, committed citizens can change the world; indeed, it’s the only thing that ever has.”



[1] Darryl John Esguerra, PH, China to sign 3 deals on joint exploration in WPS, (Oct. 29, 2018, 02:09 PM),

[2] South China Sea Arbitration (Phil. v. China), Case No. 2013-19, Award (Perm. Ct. Arb. 2016) [hereinafter “South China Sea”],

[3] United Nations Convention on the Law of the Sea art. 56, Dec. 10, 1982, 1833 U.N.T.S. 397,

[4] Antonio T. Carpio, The South China Sea Dispute: Philippine Sovereign Rights and Jurisdiction in the West Philippine Sea 224 (2017) (ebook),

[5] Joint exploration would waive PH rights over West Philippine Sea: Hilbay, ABS-CBN News (May 27, 2018, 06:31 PM),

[6] Phil. Const. art. XII, § 2, ¶ 2.

[7] Marites Dañguilan Vitug, Rock Solid: How the Philippines Won Its Maritime Case against China (2018), quoted in Mike Navallo, Arbitral ruling book author slams Duterte’s ‘defeatist’ West PH Sea stance, ABS-CBN News (July 25, 2018, 07:52 PM),

[8] Ian Brownlie, Principles of Public International Law 616 (6th ed. 2003).

[9] Emmanuel Voyiakis, Estoppel, Oxford Bibliographies (March 2012), 10.1093/obo/9780199796953-0058

[10] Case Concerning the Payment of Various Serbian Loans Issued in France (Fr. v. Serbs, Croats, and Slovenes), Judgment, 1929 P.C.I.J. (ser. A) Nos. 20/21, at 39 (July 12).

[11] Case concerning the Temple of Preah Vihear (Cambodia v. Thai.), Judgment, 1962 I.C.J. Rep. 6, at 36 (June 15) [hereinafter “Temple”],

[12] Id. at 32.

[13] In the Matter of the Chagos Marine Protected Area Arbitration (Mauritius v. U.K.), Award, ¶ 438 (Perm. Ct. Arb. 2015) [hereinafter “Chagos”], Award.pdf.

[14] Michaela Del Callar, Carpio: PHL can bring China war threat before the UN, GMA News (May 20, 2017, 04:24 PM),

[15] Joint exploration with China may be a ‘trojan horse’: ex-SolGen, ABS-CBN News (Oct. 17, 2017, 10:37 PM), solgen; Dano Tingcungco, Carpio to Duterte admin: ‘Preserve’ arbitral win vs. China, GMA News (May 25, 2018, 08:08 PM),

[16] Christian J. Tams, Waiver, Acquiescence and Extinctive Prescription, in The Law of International Responsibility 1035, 1044 (James Crawford et al. eds., 2010).

[17] Temple, supra note 11, at 62 (separate opinion by Fitzmaurice, J.).

[18] Iain MacGibbon, The Scope of Acquiescence in International Law, 31 Brit. Y.B. Int’l L. 143, 143 (1954).

[19] Ibid.

[20] Pia Ranada, Malacañang on China buildup on PH reef: What do you want us to do?, Rappler (Feb. 6, 2018, 01:49 PM),; Pia Ranada, Duterte: ‘I need China’, Rappler (Apr. 9, 2018, 05:39 PM),

[21] Joint exploration with China is co-ownership, says Duterte, (Mar. 1, 2018, 07:34 AM),

[22] Ministry of Foreign Affairs of the People’s Republic of China, Set Aside Dispute and Pursue Joint Development,, 665547/t18023.shtml (last accessed Dec. 29, 2016), quoted in Carpio, supra note 4.

[23] Carpio, supra note 4.

[24] Pia Ranada, Duterte says he would ‘go to war’ to defend Benham Rise, Rappler (Mar. 2, 2018, 09:16 PM),; Christina Mendez, We are not giving up West Philippine Sea rights – Duterte, (May 16, 2018, 12:00 AM),; Paterno Esmaquel II, Duterte vows to defend West Philippine Sea in SONA 2018, Rappler (July 23, 2018, 07:47 PM),

[25] Paterno Esmaquel II, Philippines silently files note verbale against China, Rappler (June 2, 2018, 03:04 PM),

[26] Tams, supra note 16, at 1045, construing Temple, supra note 11, at 63 (separate opinion by Fitzmaurice, J.).

[27] Chagos, supra note 13.

[28] Int’l Law Comm’n, Rep. on the Work of Its Fifty-Eighth Session, Guiding Principles applicable to unilateral declarations of States capable of creating legal obligations, with commentaries thereto, U.N. Doc. A/61/10, ¶ 1 (2006).

[29] Id. at ¶ 4.

[30] Chagos, supra note 13, ¶ 439.

[31] Sovereignty over Pedra Branca/Pulau Batu Puteh, Middle Rocks and South Ledge (Malay. v. Sing.), Judgment, 2008 I.C.J. Rep. 12, at 81, ¶ 228 (May 23),

[32] Ben O. de Vera, China to finance P743B in PH infra projects, (Oct. 29, 2018, 05:16 AM),

[33] Daxim L. Lucas, Chinese loans not tied to joint oil exploration of disputed waters, says DOF, (Oct. 10, 2018, 04:26 PM),

[34] Jess Diaz, ‘Duterte wants joint exploration with China’, (Aug. 16, 2017, 04:00 PM),

[35] PHL, China sign infra, economic cooperation agreements —DOF, GMA News (Nov. 15, 2017, 07:05 PM),

[36] Chagos, supra note 13, ¶ 445.

[37] Phil. Const. art. XII, § 2, ¶ 4.

[38] USAFFE Veterans Association, Inc. v. Treasurer of the Philippines, 105 Phil. 1030 (1959).

[39] Antonio T. Carpio, The Framework Code of Conduct, One Year After the Arbitration, Remarks delivered at the ADR Institute (July 12, 2017),