The Irish Republic has suffered a setback in its bid to put international pressure on the UK over the Sellafield nuclear plant in Cumbria.
The row centres on emissions from the Sellafield plant
The Dublin government went to the United Nations in 2001, claiming marine pollution from the site broke the UN Convention on the Law of the Sea.
But the European Commission launched a separate action, which has been upheld by the European Court of Justice (ECJ).
It ruled that Dublin's accusations should be settled within the EU.
The Irish government's complaint centred on the Sellafield MOX (mixed oxide) plant, which recycles plutonium from spent nuclear fuel.
It began operating in October 2001 and, less than a month later, Dublin took its complaints to the UN, citing environmental and health concerns regarding Sellafield's emissions.
It also complained that the UK had failed to provide Dublin with a copy of the report assessing the plant's economic justification.
Lawyers for the Republic said the UN action was justified because of alleged breaches of the UN Convention.
But the Luxembourg judges decided only they had the right to resolve a dispute between member states about an interpretation of EU law.
Reacting to the judgment, Dick Roche, the Irish Republic's Minister for the Environment, Heritage and Local Government, said: "As a result of the institution of the MOX case the UK has responded by improving its level of co-operation with Ireland.
"This has produced tangible improvements in our relationship with the UK on the nuclear issue.
"However, the significantly different perspectives between the UK and Irish governments on the continued operation of the Sellafield Nuclear Plant remain."
A spokesperson for the UK government's Department of Trade and Industry confirmed relations with the Irish over Sellafield had improved since the case began.
The spokesperson added: "In the light of these improvements, we hope that Ireland will no longer feel that it needs to have recourse to international dispute resolution, and that issues of concern to them in this area can be settled bilaterally in discussion between the two governments."