The European Commission has warned the operators of the Sellafield plant in Cumbria that they are in breach of EU rules imposing strict nuclear safeguard standards.
British Nuclear Group Sellafield (BNGSL) was told to step up controls to ensure that nuclear materials "are not diverted from the peaceful uses for which they have been declared."
The warning follows a series of Commission inspections at the Sellafield plant, which resulted in a report stating that "accounting and reporting procedures presently in place at BNGSL do not fully meet Euratom (EU) standards".
But the Commission emphasised in a statement that the warning relates solely to issues of the adequacy of those procedures: "It does not find that nuclear material was actually lost or diverted from its intended purpose and does not concern the issue of nuclear safety."
A British government spokesman said: "The UK always takes EU nuclear safeguard standards extremely seriously. The BNGSL has held its own inquiry and a review of these safeguards is now being fully implemented."