Plutonium discharged into the sea from Sellafield up to 20 years ago is being washed up on Cumbrian beaches, it has been claimed.
Sellafield is owned by British Nuclear Fuels
Norwegian researchers will officially make the claim about the nuclear reprocessing plant in a report to a conference in London, later this week.
But a spokesman for British Nuclear Fuels Ltd (BNFL), who own Sellafield, said there was "nothing new" in the Norwegian report.
The firm added it had always been up front about plutonium pollution in the sea and on the beaches, which they say poses "no risk".
Erik Martiniussen, who wrote the report for environmental group Bellona, said: "We are concerned about these plutonium concentrations because plutonium is dangerous.
"The numbers we have in the report... show the concentrations have grown in the last three years.
"That is a reason for concern."
Bellona said a 30km area of sea bed near Sellafield was contaminated by plutonium - which is a by-product of nuclear fission in reactors.
Heavy seas have disturbed the plutonium which is being brought back to the beaches by high tides and on sea spray, the report says.
Mr Martiniussen added: "These plutonium sediments have been lying in the seabed for two decades and is starting to move up onto shore."
But John Clark, BNFL head of environment, said there was no risk to health from the concentrations of plutonium.
He said: "The fact that a certain amount of material does come ashore has been known for a long time.
"There is no evidence that I am aware of that suggests this is something new and something that has changed.
"It is true that small quantities do come ashore, but... it is the conclusion of the Food Standards Agency and the Scottish Environment Protection Agency that the concentrations present no cause for concern at all."
The conference will be attended by Norwegian government officials and representatives from BNFL, the Environment Agency and the nuclear waste agency NIREX.