The European Commission told Britain to clean up its Sellafield nuclear plant today or face fines, losing patience with London's long refusal to allow full safety inspections.
The EU executive said Britain had failed to allow EU inspections to make sure nuclear material did not end up in nuclear weapons.
The Minister for Environment, Mr Martin Cullen, this evening described the Commission's statement as "hugely significant".
Mr Cullen said the Commission's findings vindicated the "substance of the case" taken by Ireland against Sellafield at the European Court of Arbitration.
"The UK operator British Nuclear Fuels (BNFL) has failed to comply with the... rules concerning accounting for nuclear material," European Commission chief spokesman Mr Reijo Kemppinen told a news conference.
BNFL had also not allowed full access to "Commission inspectors to nuclear material to check the nature and quality and quantity of the material," Mr Kemppinen said.
The Commission, which polices nuclear safety across the 15-nation bloc, has asked Britain to devise a plan to clean up Sellafield by June 1st, extending London's deadline by an extra month than originally planned.
The problem centres on B30, a series of reinforced concrete ponds that store radioactive waste under water at Sellafield.
"It is impossible to determine accurately the quantities of material stored and on the spot inspections cannot take place because of the high level of radiation and poor visibility in the part of the facility concerned," the Commission said in a statement.
If state-owned BNFL does not comply with the decision, the Commission could fine the company.
Greenpeace welcomed the decision, saying the 50-year old B30 ponds contained 1.3 million tonnes of plutonium, posing a major risk for workers and people living nearby.
"The UK Government and BNFL have prevaricated for years despite the fact that they knew there was a huge problem," said Greenpeace nuclear campaigner Ms Jean McSorley, adding that the Commission should have acted 14 years earlier.