The Government has filed an application with an international tribunal for an injunction to stop the operation of the MOX plant at Sellafield.
At the International Tribunal for the Law of the Sea, which sits in Hamburg, Government lawyers also applied to stop international movements in and around the Irish Sea of radioactive materials associated with the operation of the MOX (mixed oxide) nuclear plant.
The Government is claiming the UK violated the 1982 UN Convention on the Law of the Sea.
In a statement yesterday, Mr Joe Jacob, the Minister with responsibility for nuclear safety, said the action followed the unwillingness of the UK to suspend the authorisation of the MOX plant, as Ireland had requested when it initiated these international legal proceedings on October 25th.
It was expected BNFL would start operations at the plant around December 20th. The 21 judges of the tribunal can order a binding injunction to prevent the start-up of the plant. The tribunal will now decide on the procedure and timetable to be followed.
Mr Jacob said it was likely there would be oral hearings in Hamburg, but these were not expected to open before November 19th.
Mr Jacob said that with its request for provisional measures, Ireland had designated ambassador Alberto Szekely as ad-hoc judge at the tribunal. Ambassador Szekely was a Mexican national who participated in the negotiations of the 1982 UN convention and was a former member of the UN International Law Commission, the Minister said.
The MOX plant is designed to mix plutonium with uranium oxides to form a nuclear fuel to be burnt in reactors.
Meanwhile, the High Court in London was told that overturning the British government's decision to approve the Sellafield MOX plant would put British Nuclear Fuels Ltd (BNFL) at "a very serious commercial disadvantage".
On the final day of a judicial review of the decision, sought by Friends of the Earth and Greenpeace, counsel for BNFL insisted an EU safety directive did not compel British government ministers to "ignore economic rationality" in justifying the operation of the plant. Mr David Pannick, for BNFL, said he was instructed to tell the court: "Customers won't wait - we have competitors."