States must tackle fossil fuels, the world’s top court has ruled, and failing to prevent harm to the climate could result in them being ordered to pay reparations.

In a landmark advisory opinion published on Wednesday, the international court of justice (ICJ) said countries must prevent harm to the climate system and that failing to do so could result in their having to pay compensation and make other forms of restitution.

Presenting the 133-page document to a packed court at the Peace Palace in The Hague, the ICJ president, Yūji Iwasawa, said climate breakdown had severe and far-reaching consequences which affected natural ecosystems and people. “These consequences underscore an urgent existential threat,” he said.

The unanimous opinion covers a wide range of matters under international law. It says states are liable for all kinds of activities that harm the climate, but it takes explicit aim at fossil fuels. It says that a state’s failure to take appropriate action to protect the climate system from greenhouse gas emissions, including through the production and consumption of fossil fuels, the granting of fossil fuel exploration licences or the provision of fossil fuel subsidies, “may constitute an international wrongful act which is attributable to that state”.

Coal, oil and gas are the main cause of anthropogenic climate breakdown, but there was no mention of fossil fuels in the main text of the Paris agreement in 2015. It was not until 2023 that countries said they would “transition away” from them, and even that weaker pledge was downgraded by some governments to optional.

The court also said states were legally liable for the actions of the private sector and must regulate corporate activities that contributed to the climate crisis.

The UN instructed the ICJ to produce this document in 2023 after years of campaigning by a group of Pacific island law students and diplomacy led by the Pacific island state of Vanuatu.

It was the largest case the court has ever heard. During a two-week hearing in The Hague in December, representatives of vulnerable states told a panel of 15 judges that a handful of countries should be held legally responsible for the ongoing impacts of the climate crisis.

The world’s top greenhouse gas emitters denied they had any obligations beyond the UN framework convention on climate change (UNFCCC) and the 2015 Paris agreement. The court resoundingly rejected that argument, saying a range of other treaties applied, including the UN convention on the law of the sea, the Vienna convention for the protection of the ozone layer, the Montreal protocol, the convention on biological diversity and the UN convention to combat desertification.

Customary international law also applied, it said, including principles of sustainable development, common but differentiated responsibilities, equity, intergenerational equity and the precautionary principle. States also had a duty to work together to protect the climate, the court said, because uncoordinated action “may not lead to a meaningful result”.

The court said a clean, healthy and sustainable environment was a precondition for exercising many human rights, such as the right to life, the right to health and the right to an adequate standard of living, including access to water, food and housing.

The ICJ took aim at countries that are not part of climate change treaties, saying they still had to show that their climate policies and practices were consistent with other parts of international law. Donald Trump signed an order earlier this year to withdraw the US from the Paris agreement for the second time, and other rightwing leaders have threatened to do so as well.

The fact that the climate crisis was a global problem did not absolve individual states of responsibility, the court said, and those that have been harmed could theoretically bring climate-related legal claims against those that caused it. It may be more difficult to make a causal link than in the case of local pollution, but the court said it was not impossible and would be strengthened by existing science.

Courts could order reparations, the ICJ said, including the restoration of infrastructure and ecosystems. In cases where the damage is irreparable, compensation may be due.

Climate campaigners and representatives of vulnerable countries were delighted with the outcome. Speaking outside the court, Vanuatu’s minister for climate change, Ralph Regenvanu, said it was a milestone moment for climate justice. “It has confirmed what vulnerable nations have been saying and have known for so long,” he said. “That states do have legal obligations to act on climate change.”

He said the document would be a vital tool in upcoming climate negotiations and was likely to inspire new lawsuits. Advisory opinions are technically non-binding but are considered authoritative because they summarise existing law rather than create laws.

The ICJ is the third of four top courts to publish such a document on the climate crisis. The inter-American court of human rights (IACHR) concluded earlier this month that there was a human right to a healthy climate, and the IACHR and the international tribunal for the law of the sea said states had a legal responsibility to control greenhouse gases. The African court on human and people’s rights has only just begun the process.

Of all these, the ICJ has the widest jurisdiction and the role of harmonising and integrating international law. Vanuatu now intends to take the document to the UN general assembly to seek a resolution affirming its findings.