Samples of soil from around the villages of Newbiggin and Muncaster, some seven miles south-east of Sellafield, are far more contaminated than those taken from the "exclusion zone" around Chernobyl - site of the world's worst nuclear disaster, in 1986 - from which all the inhabitants have been evacuated.
Environmentalists claim that some of the Cumbrian fields are so radioactive that they ought to be classified as nuclear waste.
The results of the test, which were analysed at the request of Greenpeace by Bremen University and the Environment Office of Hamburg, have come at the worst time for Sellafield. Today the German Social Democratic party is negotiating with the Greens, its expected coalition partner, over whether to continue sending spent fuel from German nuclear reactors for reprocessing at Sellafield.
A decision to stop the shipments, demanded by German environmentalists, would undermine the economics of the Cumbrian complex.
Britain's Environment Agency is due to decide in 10 days whether to allow British Nuclear Fuels, which runs Sellafield, to increase radioactive discharges from the complex and to start up a new plant there to make nuclear fuel out of plutonium. The Greenpeace research could increase pressures on ministers to call in both applications for a public enquiry.
The measurements show that different samples of mud taken from the banks of the river Esk near Muncaster contained between 38 and 394 times as much americium-241 - a cancer-causing radioactive isotope - as a sample taken from the same distance south of Chernobyl.
They even contained between twice and 23 times as much of the radioactive substance as a sample taken half a mile west of the stricken Ukrainian reactor.
Samples taken from farmland at Newbiggin contained 16 to 100 times more of the isotope than the one taken the same distance from Chernobyl, and up to five times as much as the one from just a half-mile away from the Ukrainian site.
Greenpeace says that under British law these levels would require the farmland soil to be classified as nuclear waste.
Levels of cobalt-60 were also found to be higher around Sellafield than around Chernobyl, while levels of caesium-137 were about the same.
All the samples around Chernobyl were taken from within the 18-mile "exclusion zone" which is prohibited to humans and no longer used for agriculture.
"However, in the vicinity of Sellafield, people live and work, pursue agricultural activities and continue to fish and swim in the Irish Sea," said Greenpeace
It blamed a continuous build-up of radioactivity in the area since Sellafield started up in the 1950s, calling it "a slow-motion Chernobyl, an accident played out over the last decades".
Martin Forwood, campaigns co-ordinator for Cumbrians Opposed to a Radioactive Environment, Sellafield's main local opponents, said that the figures were "absolutely amazing".
"It is disgraceful that our local communities are living with radioactive rubbish so much more dangerous than the contamination that has caused evacuation of tens of thousands of people around Chernobyl," he said.
Both organisations said that all nuclear reprocessing on the site should be stopped immediately to avoid a worsening situation. But British Nuclear Fuels accused Greenpeace of scaremongering, using selective information, and insisted that radiation levels around Sellafield were not a health threat.Reuse content