MARCH 2026 | 1ST EDITION | CONSTRUCTION LAW

CUSTOMARY INTERNATIONAL LAW

The Invisible Rules Governing States

Not every law is written down.

When most people hear “international law”, they think of treaties - formal agreements that countries sign. That is only part of the picture. International law also includes rules that develop over time because states act in a certain way consistently, and because they accept that this is what the law requires. The Statute of the International Court of Justice recognises international custom as one of the main sources of international law, and the International Law Commission explains it by looking at two things: what states actually do, and whether they do it because they believe they are legally obliged to.

A simple example helps. If a foreign diplomat is in South Africa, the law generally does not treat that person in exactly the same way it would an ordinary visitor. Those protections are now written into treaties such as the Vienna Convention on Diplomatic Relations, but they also reflect long-standing international practice between states. That is the basic idea behind customary international law: rules can become legally important because countries repeatedly follow them as law, not just because they signed a document.

For South Africa, this matters more than many people realise. Section 232 of the Constitution provides that customary international law is law in the Republic unless it is inconsistent with the Constitution or an Act of Parliament. So these “invisible rules” are not just diplomatic background noise, they can form part of South African law itself. In this edition, we look at what customary international law is, how it develops, and why it still matters in practice

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THE LEGAL FRAMEWORK OF CUSTOMARY INTERNATIONAL LAW

International law does not come from one source only. Treaties are important, but they are not the whole story. Some rules develop more gradually, through the repeated conduct of states and their acceptance that such conduct is legally required. That is why customary international law is often described as the unwritten law of the international system.

HOW DOES A CUSTOM
BECOME LAW?

Two elements are usually needed.

The first is state practice, what countries actually do.

The second is a sense of legal obligation, often referred to by the Latin phrase opinio juris. Put simply, states must act in a certain way not merely because it is convenient or politically useful, but because they accept that the law requires it. The International Law Commission’s conclusions on identifying customary international law are built around this two-part approach.

This is what makes customary international law different from treaty law. A treaty is agreed in writing. Custom develops over time. It is less visible, but that does not make it less real.

WHAT KINDS OF RULES CAN BECOME CUSTOMARY INTERNATIONAL LAW?

Some of the clearest examples are rules that states rely on regularly in dealing with one another. One important example is state immunity. The United Nations Convention on Jurisdictional Immunities of States and Their Property expressly records that the jurisdictional immunities of states and their property are generally accepted as a principle of customary international law. In simple terms, that means one state is not automatically treated by another state’s courts in the same way as an ordinary private person or company.

That is what makes customary international law so important. It often provides the background rules that make international relations work, even where there is no single treaty that everyone has signed.

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HOW DO LAWYERS KNOW WHETHER A CUSTOMARY RULE EXISISTS?

This is where things become more interesting.

Customary international law is not collected in one single document, courts and lawyers have to piece it together from evidence.

They may look at official state conduct, public statements, legislation, court decisions, diplomatic correspondence, and the way countries respond to international disputes. The task is not simply to find repetition, but to identify a pattern of practice that is accepted as law.

That also explains why customary international law can sometimes be debated. A treaty clause can usually be read on a page. A customary rule often has to be proved.

DID YOU KNOW?

A rule of international law does not have to appear in a treaty to matter. If it is supported by consistent state practice and accepted as law, it may qualify as customary international law, and in South Africa, the Constitution recognises customary international law as law in the Republic unless it conflicts with the Constitution or an Act of Parliament.

A NOTE ON WHY THIS STILL MATTERS

At first glance, customary international law sounds remote and technical. In reality, it is one of the reasons international law continues to function even where no treaty provides all the answers.

Countries still need rules about how to deal with one another. Courts still need principles to apply. Governments still need legal boundaries, even when no formal agreement covers every detail. Customary international law helps fill those gaps. It gives shape to the day-to-day legal relationships between states and provides continuity across an international system that would otherwise be far more uncertain.

That is also what makes it interesting. Some of the most important international rules are not the ones announced in a dramatic signing ceremony, but the ones quietly followed over many years until they become part of the legal fabric of international life.

SOUTH AFRICA'S LEGAL POSITION

South Africa gives customary international law a direct place in its legal framework. Section 232 of the Constitution states that customary international law is law in the Republic unless it is inconsistent with the Constitution or an Act of Parliament. That is a significant constitutional choice. It means these rules are not merely persuasive or symbolic; in the right circumstances, they are part of South African law.

For a South African audience, that makes this topic more than theoretical. Customary international law may become relevant when courts interpret legal duties, when the state engages with foreign governments, and when South Africa deals with questions involving immunity or the conduct of states. The rules may be unwritten, but they are not legally invisible.

WHY THIS MATTERS OF BUSINESS

Although customary international law may sound distant from everyday business, the conduct of states often shapes the commercial world. Cross-border trade, investment, shipping, disputes involving foreign states, and dealings with state-owned entities all take place within a wider legal environment influenced by international rules, including customary ones. Rules relating to issues such as state immunity can affect where disputes are heard, how claims are pursued, and the broader level of legal certainty in international dealings.

For South African businesses operating across borders, it is therefore useful to understand that some of the rules shaping global commerce are not always found in a contract or statute book. Sometimes they come from long-standing international practice - and can still have real legal and commercial consequences.

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