"We are taking this action to demand that British authorities comply with their responsibilities," Energy Commissioner Loyola de Palacio told a news conference.

"We have to ensure that EU citizens are appropriately protected, that they are informed, that they have a guarantee that all nuclear power stations within the EU are functioning appropriately," she said.

The problem centres on B30, a subdivided concrete pool that is filled with radioactive waste, corroded fuel elements and sludge. The EU executive wanted a plan by June 1 that guaranteed the nuclear materials stored in he pool would be "properly accounted for" by UK operator British Nuclear Fuels (BNFL).

The Commission, which policies nuclear safety across the 25-nation bloc, also wanted guarantees the site would be completely accessible to Commission safety inspectors.

De Palacio said the information provided was not enough and had not been formally approved by the relevant UK authorities.

But she said she was confident that Britain now would respond to the increased pressure. Talks between the two sides could lead the Commission to withdraw its court case.

"I am confident the UK will put forward the adequate commitments and plans," de Palacio said.

A PROBLEM OF ACCOUNTING

A spokeswoman at Britain's EU representation in Brussels emphasised the Commission's concerns were related to accounting for what is done with the material in the pond - not environmental risks.

"We recognise that this is a problem," said Emma Lockwood. "It's not about security and it's not about the environment. It's about accounting for what's in the pond."

The Greens/EFA group in the European Parliament welcomed the decision but said it was overdue.

"Although inspection visits have been conducted on an annual basis for 15 years, the problem remains the same: inspectors appear to be in the dark over exactly what is in the B30 pond," said Member of Parliament Caroline Lucas in a statement.

"As a result, nobody can be sure whether plutonium or uranium has been diverted for weapons use. This is particularly worrying given that it only takes a few kilograms of plutonium to make a nuclear bomb."

De Palacio also said she did not believe the plant posed any danger to EU citizens because British authorities were carrying out inspections.

This is the first time the EU executive has taken a member country to the European Court of Justice over a nuclear safety issue. No other nuclear plants in the 25-nation bloc posed similar concerns, de Palacio said.