"The shipment of such materials through the Irish Sea represents an unacceptable risk to the environment of Ireland and the health and economic wellbeing of its population," said Environment Minister Martin Cullen in a statement.

"There is also the enhanced risk of the shipments being the target of a terrorist attack or the materials being diverted into the hands of terrorists."

The MOX fuel is being returned to state-owned British Nuclear Fuels after Japan's Kansai Electric Power Co Inc discovered that data for a 1999 shipment from Britain had been deliberately falsified.

The planned route of the shipment has been kept secret for security reasons and BNFL insists all necessary safety measures have been taken.

"The Irish government is concerned to ensure that vessels carrying such materials do not pass through waters under Irish jurisdictions, and there are currently assurances from the UK authorities that they will not do so," said Cullen.

Sellafield, 110 miles (180 km) across the Irish Sea from Ireland on England's northwest coast, has been a long-running source of friction between the two countries, with repeated calls from the Irish government for its closure.

Ireland says the plant pollutes the Irish Sea and presents a serious risk from accident or terrorist attack, and fears have been heightened since the September 11 attacks on the U.S.

Britain first established nuclear facilities at Sellafield, formerly called Windscale, in the 1940s, and the world's first commercial nuclear power station opened there in 1956.