2.3 Flag State jurisdiction under UNCLOS
As a starting point, UNCLOS establishes exclusive flag State jurisdiction as the default rule governing vessels on the high seas(1)Article 92(1) UNCLOS. The concept of flag State jurisdiction constitutes a central jurisdictional link for the regulation of vessel‑based activities on the high seas and in ABNJ. This means that vessels operating on the high seas are subject to the authority of the State whose flag they are entitled to fly, departing from it only in narrowly defined exceptions provided for in UNCLOS and certain other rules of international law(2)Articles 92(1) and 100-110 UNCLOS. This way of allocating authority reflects foundational principles of the law of the sea, such as freedom of navigation and respect for State sovereignty(3)Articles 87 and 89 UNCLOS. It is this exclusive jurisdictional link that provides the basis for imposing environmental assessment obligations on flag States for vessel-based activities in ABNJ.
The exclusive jurisdiction enjoyed by flag States is often premised on the understanding that the attribution of nationality puts the flag State in the optimal position to regulate and control its vessels(4)Ibid n57, page 33 . The granting of nationality, which is to be conditioned on the existence of a genuine link between the flag State and the vessel, establishes both a legal and administrative connection capable, in principle, of supporting effective regulatory oversight. Article 92 of UNCLOS reinforces the notion of exclusivity or supremacy of flag State jurisdictions through establishing that ships shall sail only under one flag(5)Article 92 UNCLOS. By conferring exclusive jurisdiction on the flag State, UNCLOS makes it clear that in ABNJ, the flag State bears the central role in governing vessel conduct, which includes ensuring compliance with international rules, and taking enforcement action where necessary(6)Article 92 UNCLOS.
It is deemed important to emphasise that, under UNCLOS, flag State jurisdiction is not exclusively permissive. Rather, it is accompanied by a positive obligation requiring that the flag State exercises “effective jurisdiction and control”(7)Article 94 UNCLOS over the vessels that are flying its flag. By allocating jurisdiction and control over a vessel to its flag State, UNCLOS provides the juridical basis on which the flag State’s international responsibility is engaged, particularly where the State Party fails to exercise due diligence in regulating and enforcing applicable obligations. In environmental matters, this means that a flag State cannot simply claim jurisdiction as a formal right, but the flag State must use that jurisdiction to regulate, supervise, and control the conduct of vessels to prevent environmental harm(8)Article 94(1) UNCLOS.
While UNCLOS does not provide an explicit definition of a flag State(9) Nguyen, Lan Ngoc., ‘Flag States’ Rights and Obligations under the Law of the Sea’ (2023), Cambridge University Press, Chapter 3.1, page 67, a starting point is Article 90’s definition that “Every State, whether coastal or land-locked, has the right to sail ships flying its flag on the high seas”(10)Article 90 UNCLOS. Article 91(1) further establishes that a ship has the nationality of the State whose flag it is entitled to fly, and it requires each State to determine the conditions for granting its nationality to ships(11)Article 91(1) UNCLOS. This discretion placed upon the flag State is paired with the requirement that there must be a “genuine link between the State and the ship”(12)Article 91(1) UNCLOS. The requirement of a genuine link underscores that flag State jurisdiction is intended to be accompanied by substantive regulatory responsibility rather than being a purely formal registration, and it has also been argued to have been included as “a means of tackling the use of ‘flags of convenience’”(13)Richard Barnes, R., chapter 14 ‘Flag States’ (June 2016) The Oxford Handbook of the Law of the Sea, Donald Rothwell (ed.) et al., page 306. UNCLOS does not define what constitutes a genuine link, and it has been the topic of many scholarly articles(14)See Moira L McConnell, ‘Business as Usual: An Evaluation of the 1986 United Nations Convention on the Conditions for Registration of Ships’ (1987) 18 Journal of Maritime Law and Commerce, page 435; and see also Robin Churchill and Christopher Hedley, ‘The Meaning of the “Genuine Link” Requirement in Relation to the Nationality of Ships’ (International Transport Workers' Federation, 2000), https://orca.cardiff.ac.uk/id/eprint/45062/1/ITF-Oct2000.pdf accessed the 19.04.2026, but it can be argued to reflect an expectation that the attribution of nationality should support some sort of meaningful regulatory control by the flag State.
Although UNCLOS does not define what constitutes a ship, ITLOS has interpreted this concept broadly. In the case of M/V Saiga (No 2)(15)M/V ‘Saiga’ (No. 2) (Saint Vincent and the Grenadines v Guinea) (Judgment) ITLOS Reports 1999, paragraph 106, ITLOS held that “the ship, everything on it, and every person involved or interested in its operations”(16)Ibid n90 is a unit linked with the flag State. This interpretation has subsequently been confirmed in later jurisprudence(17)See M/V “Virginia G” Case (Panama v Guinea‑Bissau) (Judgment) (International Tribunal for the Law of the Sea, Case No 19, 14 April 2014), paragraph 120, and the expansive definition confirms that flag State jurisdiction attaches to the ship as a legal unit constituted through nationality, rather than solely to the geographic location of the vessel and its activities.
Flag State obligations are further reinforced by Part XII of UNCLOS, containing the responsibilities with respect to the environment. In Article 192, UNCLOS subjects all State Parties to an obligation to protect and preserve the marine environment(18)Article 192 UNCLOS. Article 194 requires State Parties to take all measures necessary to prevent, reduce, and control pollution of the marine environment from any source(19)Article 194 UNCLOS. Of particular relevance for the ABNJ is Article 194(2), which requires that State Parties ensure that activities under their “jurisdiction or control” are conducted so as not to cause damage by pollution to other States and their environment, and to prevent pollution from spreading beyond areas where they exercise sovereign rights(20)Article 194(2) UNCLOS. With regards to vessel‑source pollution, Article 194(3)(b) underscores that measures must include those designed to minimise pollution from vessels, while Article 211 requires States to adopt laws and regulations relating to vessel‑source pollution that are at least as effective as “generally accepted international rules and standards”(21)Article 211 UNCLOS formulated through IMO as the competent international organisation. Seen together, these provisions establish that flag State jurisdiction in the shipping context is expected to be exercised through domestic implementation and enforcement of internationally agreed standards.
In the context of international shipping, these obligations are most directly engaged in relation to vessel‑source pollution. UNCLOS treats pollution from vessels as a distinct category of marine pollution, encompassing “measures for preventing accidents and dealing with emergencies, ensuring the safety of operations at sea, preventing intentional and unintentional discharges, and regulating the design, construction, equipment, operation and manning of vessels”(22)Article 194(3)(b) UNCLOS. While UNCLOS does not elaborate technical discharge standards itself, it establishes the normative framework within which vessel‑source pollution must be prevented, reduced, and controlled. Regulatory authority is accordingly channelled through generally accepted international rules and standards developed within the IMO. The International Convention for the Prevention of Pollution from Ships (‘MARPOL’)(23)International Convention for the Prevention of Pollution from Ships (adopted 2 November 1973; entered into force 2 October 1983, London) 12 ILM 1319 (MARPOL) constitutes the principal instrument through which these standards are operationalised, while Articles 211, and 217 of UNCLOS locate primary responsibility for their implementation, supervision, and enforcement with the flag State(24)Articles 211 and 217 UNCLOS. UNCLOS further makes it clear that the flag State’s role is not limited to rulemaking but extends to enforcement. Article 217 requires flag States to ensure that vessels flying their flag comply with applicable international rules and standards for the prevention, reduction, and control of pollution from vessels(25)Article 217 UNCLOS. The combined effect of Articles 94, 211 and 217 is therefore that rules on vessel-source pollution are primarily overseen by the flag State, which has exclusive authority, and the flag State must control its vessels effectively and enforce the international standards.