The groups said they were seeking judicial review of the government's decision last week to allow British Nuclear Fuels to begin operations at its mixed oxide (MOX) plant."The government's decision to allow the MOX plant to open is dangerous, uneconomic and perverse," said Charles Secrett, director of Friends of the Earth in statement.
"It makes the world an even more dangerous place."
The decision prompted a storm of protest in Ireland where Irish Prime Minister Bertie Ahern promised last weekto raise the issue with his British counterpart Tony Blair and to take legal action to halt the plant.
Ireland fears a nuclear accident at Sellafield, some 60 miles (96 km) from the Irish coast, and since the suicide attacks on the United States on September 11, the threat of a similar assault on the plant has been raised.
Britain said it gave the go-ahead to the project because the economic case for opening it outweighed the social and environmental detriments.
The plant takes plutonium from BNFL's neighbouring THORP reprocessing plant to make MOX fuel for nuclear power stations.
The MOX plant was completed in 1996 but has lain idle ever since. There have been five reviews of its fate in the last four years.
Green groups argue the economic costs of the project have been distorted as the government has disregarded construction costs. They say there is insufficient evidence that potential customers will materialise.