The Permanent Court of Arbitration (PCA) in the Hague rejected on Thursday India's objections to a Pakistan-initiated procedure over water use in the Indus River basin, reopening a procedure that had been blocked for many years.
India called the arbitration proceeding illegal as a neutral expert was also looking at the issue and the World Bank-brokered treaty prohibits parallel proceedings.
The South Asian neighbours have been arguing over hydroelectric projects on the shared Indus River and its tributaries for decades, with Pakistan complaining that India's planned hydropower dams in upstream areas will cut flows on the river which feeds 80% of its irrigated agriculture.
To resolve the dispute, Pakistan sought resolution through PCA arbitration proceedings in 2016, prompting India to request that the World Bank appoint a neutral expert under the terms of the treaty. India has boycotted The Hague court proceedings and questioned the competence of the court.
"In a unanimous decision, which is binding on the Parties and without appeal, the Court rejected each of the objections raised by India and determined that the Court is competent to consider and determine the disputes set forth in Pakistan's Request for Arbitration," the court said in a statement.
It gave no details on when and how the case will continue, but added that it will address the interpretation and application of the bilateral Indus Water Treaty, notably the provisions on hydro-electric projects, as well as the legal effect of past decisions of dispute resolution bodies under the Treaty.
A spokesperson for India's foreign ministry, Arindam Bagchi, said India's "consistent and principled position has been that the constitution of this so-called court of arbitration is in contravention of the clear letter and spirit of the Indus Water Treaty".
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He said India was participating in the proceedings of the neutral expert, which he called "the only treaty-consistent proceedings at this juncture".
"Legal sophistry" will not compel India to participate in the proceedings of the PCA, Bagchi said.
India says the construction of its Kishanganga and Ratle Hydro Electric projects is allowed by the treaty.
Pakistan's Foreign Office said that it remained fully committed to the implementation of the Indus Water Treaty and its settlement mechanism, which it termed a "foundational agreement" between the two countries.
"We hope that India would also implement the Treaty in good faith," Foreign Office spokeswoman Mumtaz Zahra Baloch said in a statement on Thursday night.