Customary International Law

Scope: Customary international law in environmental matters: how a rule becomes custom, which environmental rules are customary, and how Indian courts use them.

In one minute

A treaty binds only the States that sign it. Customary international law is different. It binds all States, because they follow a practice and believe the law requires it.

In environmental law, the no-harm rule is the standard example. Indian courts also treat some environmental principles as customary law, and they can then apply them at home.

Exam focus

Frequency: No data
Usually asked as: No data

Legal provision

Article 38(1)(b) of the Statute of the International Court of Justice: lists international custom as evidence of a general practice accepted as law.
Article 51(c) of the Constitution of India: the State shall endeavour to foster respect for international law and treaty obligations.
Vellore Citizens Welfare Forum (1996): the Supreme Court held that sustainable development is part of customary international law.
It added that a customary rule not contrary to domestic law is deemed incorporated in it.

Meaning and definition

A rule of customary international law needs two things.

A general and consistent practice of States.
A belief that the practice is required by law, called opinio juris (belief that the practice is legally required).

Practice without that belief is only habit. A belief without practice is only a wish.

Detailed explanation

Step 1: why custom matters in environmental law

Treaties take years to agree and bind only their parties. Custom fills the gap. A State that has not joined any environmental treaty is still bound by the customary rules.

Step 2: the no-harm rule as custom

The Trail Smelter tribunal stated that no State may use or permit the use of its territory to cause serious injury to another State. That statement needed no treaty. Stockholm Principle 21 and Rio Principle 2 later repeated it, which shows States accepting it as law.

Step 3: sustainable development as custom in India

In Vellore, the Supreme Court said it had no hesitation in holding that sustainable development is part of customary international law. It added that its salient features are yet to be finalised by international jurists.

Step 4: customary rules inside Indian law

The Court noted that a customary rule not contrary to municipal law is deemed incorporated in domestic law, and courts follow it. It then held that, once the precautionary and polluter pays principles are accepted as customary law, they can be accepted as part of domestic law.

Example. A tannery cluster in Tamil Nadu pollutes a river. No statute spells out the polluter pays idea for the case. The High Court applies it as part of Indian law, because the Supreme Court has accepted it.

Step 5: how soft law becomes custom

A soft-law declaration does not bind. If States keep acting on it and treat its rule as legally required, the rule can harden into custom. This is how the ideas of Stockholm shaped later law.

Common mistakes

Saying custom needs a treaty or a written text. It does not.
Leaving out opinio juris and naming only State practice.
Saying customary rules override Indian statutes. They apply in India only if not contrary to domestic law.

Leading case laws

Vellore Citizens Welfare Forum v. Union of India
Facts. Tanneries in Tamil Nadu discharged untreated effluent, polluting land and drinking water around the Palar river.

Principle. Sustainable development is part of customary international law. A customary rule not contrary to municipal law is deemed incorporated in domestic law.

Why it matters. It is the leading Indian authority for using customary rules in environmental cases.
Trail Smelter Arbitration (United States v. Canada)
Facts. Fumes from a Canadian smelter damaged land in Washington State.

Principle. No State may use or permit the use of its territory to cause serious injury to another State, shown by clear and convincing evidence.

Why it matters. It states the no-harm rule that is treated as customary law.

How to write this answer in the exam

5-mark answer

Meaning: State practice plus opinio juris.
No-harm rule as custom.
Vellore on sustainable development.

10-mark answer

Meaning and the two elements.
Difference from a treaty.
No-harm rule: Trail Smelter, Stockholm and Rio.
Vellore: incorporation into domestic law.
How soft law hardens into custom; conclusion.

Key points to remember

Custom = general State practice + opinio juris.
It binds all States without a treaty.
No-harm rule: customary, from Trail Smelter.
Vellore: sustainable development is customary; a customary rule not contrary to domestic law is deemed incorporated.

Quick self-test

1. What are the two elements of custom?

General State practice and opinio juris.

2. Which case held sustainable development to be customary international law?

Vellore Citizens Welfare Forum v. Union of India.

3. When does a customary rule apply in India?

When it is not contrary to domestic law.