The International Court of Justice in The Hague is understood to have decided that Israel's West Bank barrier is illegal and should be removed.
The court has now started reading out its ruling. Leaked reports say it will call on the United Nations to consider what action is required.
Israel insists the barrier is needed to keep out West Bank militants. The Palestinians consider it a land grab.
The court's decision is not binding, but can serve as a basis for UN action.
The ICJ began issuing its ruling at 1300GMT, in a reading expected to take more than two hours.
In the first part of the reading, the court rejected Israel's argument that it has no right to rule on the legality of the barrier.
Several countries, including the US and UK, had argued the court should stay out of the issue, warning that any opinion it gives could interfere with the Middle East peace process.
The White House reiterated its opinion as the ruling was under way, saying the ICJ was not the appropriate forum for resolving a political issue. But the court has rejected that argument.
According to a leaked copy of the report, seen by Reuters and reported in the Israeli press, the court has decided the construction of the barrier is against international law.
The current route of the barrier cannot be justified for military needs, the leaked report says.
"The wall, along the route chosen, and its associated regime, gravely infringe a number of rights of Palestinians residing in the territory occupied by Israel, and the infringements resulting from that route cannot be justified by military exigencies or by the requirements of national security or public order," the leaked report says.
The ICJ has said it will publish the official report on its website after the ruling.
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