Article 253 of the Constitution of India states that the Parliament has the power to make any law for the whole or any part of the territory of India for implementing any treaty, agreement or convention.
Generally, therefore, a legislation is required for implementing the international conventions, unlike the position in the United States of America where the rules of international law are applied by the municipal courts on the theory of their implied adoption by the State, as a part of its own municipal law.
Article VI, Cl. (2) of the U.S. Constitution reads as follows:
…all treaties made, or which shall be made, under the authority of the United States, shall be the supreme law of the land, and the judges in every State shall be bound thereby, anything in the Constitution or laws of any State to the contrary not-withstanding.
In the United States, however, it is open to the courts to supersede or modify international law in its application or it may be controlled by the treaties entered into by the United States. But, till an Act of Congress is passed, the Court is bound by the law of nations, which is part of the law of the land. Such a supremacy clause is absent in our Constitution.
Courts in India would apply the rules of International law according to the principles of comity of Nations, unless they are overridden by clear rules of domestic law.
In the case of Jolly George Varghese v. Bank of Cochin (1980), the Court applied the above principle in respect of the International Covenant on Civil and Political Rights, 1966 as well as in connection with the Universal Declaration of Human Rights. India has ratified the above mentioned covenants, hence, those covenants can be used by the municipal courts as an aid to the Interpretation of Statutes by applying the Doctrine of Harmonization. But, certainly, if the Indian law is not in conflict with the International covenants, particularly pertaining to human rights, to which India is a party, the domestic court can apply those principles in the Indian conditions.
The Interpretation of International Conventions is governed by Articles 31 and 32 of the Vienna Convention on the Law of Treaties of 1969.
Article 51 of Indian Constitution
Article 51 of the Directive Principles of State Policy, which falls under Part IV of the Indian Constitution, reads as under:
Art. 51. The State shall endeavor to
a) promote international peace and security;
b) maintain just and honourable relations between nations;
c) Foster respect for international law and treaty obligation in the dealings of organised peoples with one another; and
d) Encourage settlement of international disputes by arbitration.
Article 51, as already indicated, has to be read along with Article 253 of the Constitution. If the parliament has made any legislation which is in conflict with the international law, then Indian Courts are bound to give effect to the Indian Law, rather than the international law.
However, in the absence of a contrary legislation, municipal courts in India would respect the rules of international law.
In His Holiness Kesavananda Bharati Sripadavalvaru v. State of Kerala (1973), it was stated that in view of Article 51 of the Constitution, the Court must interpret language of the Constitution, if not intractable, in the light of United Nations Charter and the solemn declaration subscribed to it by India.
In Apparel Export Promotion Council v. A. K. Chopra (1999), it was pointed out that domestic courts are under an obligation to give due regard to the international conventions and norms for construing the domestic laws, more so, when there is no inconsistency between them and there is a void in domestic law.
Reference may also be made to the Judgments of this Court in Githa Hariharan (Ms) and another v. Reserve Bank of India and another (1999) 2 SCC 228, R.D. Upadhyay v. State of Andhra Pradesh and others (2007) 15 SCC 337 and Peoples Union for Civil Liberties v. Union of India and another (2005) 2 SCC 436.
In Vishaka and others v. State of Rajasthan and Others (1997) 6 SCC 241, this Court under Article 141 laid down various guidelines to prevent sexual harassment of women in working places, and to enable gender equality relying on Articles 11, 24 and general recommendations 22, 23 and 24 of the Convention on the Elimination of All Forms of Discrimination against Women. Any international convention not inconsistent with the fundamental rights and in harmony with its spirit must be read into those provisions, e.g., Articles 14, 15, 19 and 21 of the Constitution to enlarge the meaning and content thereof and to promote the object of constitutional guarantee.
 (1980) 2 SCC 360
 (1973) 4 SCC 225
 (1999) 1 SCC 759