The International Court of Justice in The Hague has ruled that the barrier Israel is building in the West Bank breaches international law and cannot be justified by Israel's security concerns.
"The wall... cannot be justified by military exigencies or by the requirements of national security or public order," said Judge Shi Jiuyong of China. "The construction of such a wall accordingly constitutes breaches by Israel of its obligations under the applicable international humanitarian law."
"Israel ... has the obligation to cease forthwith the works of construction of the wall being built by it in the Occupied Palestinian Territory, including in and around East Jersualem," the UN's highest legal body said.
"The Court finds further that Israel has the obligation to make reparation for the damage caused to all the natural or legal persons concerned," it said.
Israel has said it will not accept a ruling from the court on the network of fences, ditches and concrete wall it says it is building to keep out bombers. Palestinians call it a land grab that destroys their hopes of a viable state.
The ruling agreed saying the barrier could become tantamount to annexation of Palestinian land if it is completed and that it impeded the Palestinian's right to self-rule.
"The Court considers that the construction of the wall and its associate regime creates a 'fait accompli' on the ground that could well become permanent, in which case, and notwithstanding the formal characterisation by Israel, it would be tantamount to de facto annexation," the UN court said.
"That construction, along with measures previously taken, thus severely impeded the exercise by the Palestinian people of its right to self-determination," the court said in a ruling on the legality of the barrier.
The UN's highest court also said in the 400-page ruling that Israeli settlements on occupied territory breached international law.
Israel said it would disregard the decision and repeated that the barrier was vital to keeping out suicide bombers.
The US government also dismissed the UN ruling. "We do not believe that that's the appropriate forum to resolve what is a political issue. This is an issue that should be resolved through the process that has been put in place, specifically the road map," White House spokesman Mr Scott McClellan said.
The Labour Party's foreign affairs spokesman, Mr Michael D Higgins, called on the Government to "use the great influence it has established in international affairs over recent years, through its membership of the United Nations Security Council and the Presidency of the European Union, to seek implementation of the legal opinion." If Israel did not comply, he said, the Government should seek sanctions against Israeli authorities.
Ahead of the ruling today, the Ireland Palestine Solidarity Campaign (IPSC) issued a statement demanding the international community take measures to make Israel comply with the Court's finding.
"The humanitarian and political consequences of the wall are clear. Hundreds of thousands of Palestinian civilians experience limitations in their freedom of movement, lose access to their land and water and become isolated from their sources of income, schools and hospitals.
"The wall leads to incorporation of most of the illegal settlements in occupied territories into Israel, resulting in the de facto annexation of major portions of Palestinian land," the statement said.
The IPSC also condemned the EU response to a UN resolution last October describing the wall as being "in contradiction to relevant provisions of international law".
The IPSC called on the Dutch presidency to galvanise member states to develop effective measures against the wall", including the suspension of the Euro-Mediterranean Association Agreement.
A European Commission spokesman said the court finding supported the EU's view and called for Israel to remove the barrier.