Accounting and reporting procedures at the Sellafield nuclear reprocessing plant in Cumbria have been criticised by European Commission (EU) inspectors.
The EU says Sellafield must tighten up procedures
While the EU says no nuclear material has gone missing, it says operator British Nuclear Group (BNG) is in breach of strict safeguard standards.
They must ensure nuclear materials "are not diverted from the peaceful uses for which they have been declared."
A 2000 inquiry at the plant found safety records had been falsified.
In a statement BNG said it would study the inspection report carefully.
The EU report said "accounting and reporting procedures presently in place do not fully meet EU standards".
An EU spokesman said: "The commission has also requested BNG to implement the appropriate remedies ... to ensure the adequate quality of its system of accounting for nuclear material."
A BNG spokesman said: "We have not yet received any formal notification of such a warning.
"Naturally, we will study the content of any such decision, if and when we receive notification of it from the commission, and consider the appropriate response for the company at that time."
A government spokesman said: "The UK always takes EU nuclear safeguard standards extremely seriously.
"BNG has held its own inquiry and a review of these safeguards is now being fully implemented."
The Sellafield site is one of the largest nuclear engineering centres in the world.
Last year, Sellafield received a separate commission warning about the handling of nuclear waste disposal at the plant.
Under the 1957 Euratom Treaty it is up to EU inspectors to check accounting records of the nuclear material and compare them with the results of on-the-spot inspections.
The EU has said that the main purpose of such inspections is to make sure the nuclear material used is not diverted from peaceful and non-military uses.