One principle in international maritime law is the principle of No More Favourable Treatment (NMFT).
Every day, Port State Control Officers (PSCO’s) around the world inspect ships flying the flags of both Parties and non-Parties to IMO conventions, ensuring that vessels are not placed at a competitive advantage simply because their flag State has chosen not to ratify a particular treaty.
For most maritime lawyers, NMFT is simply accepted as part of the PSC framework.
Yet it raises an interesting legal question:
Has the principle of No More Favourable Treatment evolved beyond its treaty origins and become a rule of customary international law?
I would argue that there is a compelling legal argument that it has.
How does treaty law become customary international law?
Unlike treaty law, customary international law binds all States (subject to recognised exceptions such as the persistent objector doctrine), irrespective of whether they are parties to a particular treaty.
Article 38(1)(b) of the Statute of the International Court of Justice identifies international custom as:
“evidence of a general practice accepted as law.“
(See: Statute of the International Court of Justice)
The modern methodology for identifying customary international law is today found in the International Law Commission’s Draft Conclusions on Identification of Customary International Law (2018). The ILC explains that two constituent elements must generally be established:
- a general practice, and
- acceptance of that practice as law (opinio juris).
- (See: Draft Conclusions on Identification of Customary International Law (2018) )
The ILC’s work—prepared by Sir Michael Wood, former Legal Adviser to the UK Foreign & Commonwealth Office—has become the principal framework used by governments, courts and scholars when analysing customary international law. The UN General Assembly welcomed the Draft Conclusions and encouraged their widest dissemination.
Likewise, the International Court of Justice recognised in the North Sea Continental Shelf Cases (1969) that treaty provisions may, through widespread State practice accompanied by opinio juris, crystallise into customary international law.
(See: North Sea Continental Shelf Cases (1969))
The treaty origins of NMFT
Historically, NMFT developed as a treaty mechanism.
The purpose was straightforward.
If only ships flying the flags of States Parties were subject to international requirements, shipowners could simply register vessels under the flags of non-Parties and thereby obtain a commercial advantage.
To prevent this, the IMO gradually incorporated NMFT into virtually every major convention relying on Port State Control.
Examples include:
- MARPOL – Article 5(4)
- SOLAS – Article I(3)
- STCW – Article 10(5)
- Ballast Water Management Convention – Article 3(3)
- Hong Kong Convention – Article 3(4)
- Maritime Labour Convention – Article V(7)
Today, NMFT has become the common denominator of modern Port State Control.
The first element: General State practice
Applying the International Law Commission’s methodology, the first question is whether there exists sufficiently widespread and representative State practice.
There is a compelling argument that this requirement is met as IMO rules in enforced in accordance herewith every day.
Port State Control authorities operating under the:
- Paris MoU
- Tokyo MoU
- Indian Ocean MoU
- Mediterranean MoU
- Black Sea MoU
- Caribbean MoU
- Abuja MoU
- Riyadh MoU
- Viña del Mar Agreement
- as well as the United States Coast Guard’s national Port State Control programme,
routinely inspect ships from States that are not Parties to the relevant convention, ensuring that no more favourable treatment is afforded to those vessels.
Few principles of international maritime law enjoy such widespread and continuous administrative application.
The second element: Opinio juris
The second constituent element is whether States accept the practice as legally required.
Here too, the argument is persuasive.
For more than four decades, States have repeatedly negotiated, adopted and ratified IMO conventions containing NMFT clauses.
They have then implemented those provisions through domestic legislation and Port State Control procedures.
Equally significant is why States apply the principle.
NMFT is consistently justified not merely as sound policy but as a legal necessity.
Without it, shipowners could gain a competitive advantage by selecting flags of non-Parties, thereby undermining:
- maritime safety;
- environmental protection;
- labour standards; and
- fair competition.
In other words, States appear to regard NMFT as legally necessary to preserve the integrity of the international maritime regulatory system.
Does NMFT conflict with pacta tertiis?
The principal objection is obvious.
Articles 34 and 35 of the Vienna Convention on the Law of Treaties (VCLT) provide that treaties neither create obligations nor confer rights upon third States without their consent.
(See: Vienna Convention on the Law of Treaties (UN Treaty Collection) )
How then can a ship flying the flag of a non-Party be required to comply with treaty standards?
The better view is that NMFT does not bind the non-Party flag State.
Rather, it reflects the sovereign right of the port State to regulate access to its own territory cf. UNCLOS art. 2.
Ships enter foreign ports voluntarily.
When they do, they become subject to the conditions established by the port State for entry and use of its ports.
The port State is therefore not enforcing treaty obligations against another State; it is exercising its territorial jurisdiction.
Seen in this light, NMFT does not undermine the principle of pacta tertiis. It represents an application of territorial sovereignty.
Has NMFT crossed the threshold?
Ultimately, only an international court can authoritatively determine whether NMFT has attained the status of customary international law.
Nevertheless, applying the methodology adopted by the International Law Commission suggests that the principle exhibits many of the recognised characteristics of customary international law:
- consistent State practice;
- widespread geographical application;
- repeated incorporation into successive multilateral treaties;
- routine implementation by maritime administrations worldwide; and
- broad acceptance as legally necessary.
Perhaps the strongest argument is this:
NMFT is no longer merely another treaty provision. It has become the default legal architecture upon which modern Port State Control operates.
Concluding remarks
International law is not static.
History shows that principles initially developed within treaties may, through decades of consistent State practice and legal acceptance, evolve into rules of customary international law.
Whether No More Favourable Treatment has already crossed that threshold remains an open legal question.
However, applying the International Law Commission’s methodology suggests that the argument deserves serious consideration.
If accepted, NMFT would represent an excellent example of how international maritime law continues to evolve—not solely through new treaties, but through the gradual development of customary international law itself.
Further Reading
International Law Commission
Oxford University Press
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