"The MOX plant is not only an environmental threat and a potential terrorist target, but simply does not make business sense," Greenpeace executive director Stephen Tiddle told reporters outside the court.

The green lawyers said the government had not showed sufficient economic justification for the plant, as required by tEU law, because its 470 million pound ($690 million) cost was not taken into account when assessing its commercial viability.

It was also argued there was insufficient evidence there would be enough customers for the fuel - a mixture of highly-toxic plutonium and uranium oxides.

Greenpeace and Friends of the Earth say launching the plant would lead to more plutonium production at Sellafield.

Some nuclear experts believe it would be relatively easy to extract plutonium, which could be used in a nuclear device, from MOX fuel rods.

BNFL said it was awaiting the outcome of the judicial review before the plant.

BENEFITS V COSTS

A government-commissioned study, conducted by consulting firm Arthur D. Little and published in July, said the plant would deliver net financial benefits of 216 million pounds.

The report also said the cost of not opening the plant could run into hundreds of millions of pounds largely due to potential loss of future contracts for THORP, BNFL's nuclear reprocessing plant.

The Sellafield MOX Plant has lain idle since 1996 because regulatory approval to start-up was repeatedly withheld over fears it would not make any money.

BNFL says the MOX plant can be profitable and that it already boasts a healthy order book from overseas customers. The group also dismisses suggestions it would be easy to extract plutonium from MOX to make a nuclear device.

More legal battles face BNFL, which had its partial privatisation shelved in 2000 after a scandal erupted when it was discovered staff had falsified data on pilot batches of MOX fuel sent to customers.