Sellafield has been warned it could face stiff penalties for not meeting stringent European Union (EU) rules on nuclear waste.
Sellafield has been given until June to comply with the EU's demands
The Cumbrian nuclear reprocessing plant has been given until 1 June to come up with an accounting plan on how spent nuclear fuel has been processed.
The EU has told British Nuclear Fuels (BNFL), Sellafield's major operator, that the situation is "unacceptable".
The UK government has said that BNFL officials are co-operating with the EU.
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.
Currently the spent fuel is held in a "pond" at the Sellafield site.
The European Commission chief spokesman Reijo Kemppinen said: "BNFL is failing to comply with the rules concerning accounting for nuclear material, and the access of Commission inspectors to nuclear material to check the nature and quality of the material."
WHAT SELLAFIELD HAS TO DO
Adequate plan accounting for nuclear material
Physical access to the facilities concerned
Six monthly report on progress implementing the plan
Source: EU Commission
A statement went on to say that inspectors have for a number of years informed BNFL the nuclear material in question could not be inspected properly, in contravention of the Euratom Treaty.
"It is irradiated fuel. In accounting terms, 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," it said.
The statement added: "Despite its commitments, BNFL has so far failed to come up with a formal action plan or adopt the measures needed to put an end to the infringement once and for all."
Under the 1 June deadline there has to be an overall plan ensuring adequate accounting for the nuclear material in question.
Also, there must be physical access to the facilities and every six months the EU has to receive a report on progress implementing the plan.
A spokesman for the UK government has said it was aware action was needed.
"We know there is a problem and we are very open about that. The government has already set up a d-commissioning agency to clean up at Sellafield, but this is not something we will be rushed into.
"The Commission is surely not suggesting that the UK authorities may be diverting this material for non-peaceful uses, which is the real purpose of their checking. There is no suggestion of any leakage."
Jean McSorley of the environmental campaign group Greenpeace backed the commission warning, but she said Brussels was missing the point about nuclear energy.
"This dispute smacks of a row between the UK and the EU over responsibility for enforcing safety, rather than being about protecting the health and safety of workers at Sellafield and the people who live around the site," she said.
In 2000 an investigation found that safety records at the 34-year-old plant had been systematically falsified.