In an official complaint to Michael Meacher, the environment minister, they say human health and the environment could be at risk from a 50-fold increase in some discharges over just four years.
Their joint demand for cuts in discharges, already rejected by Mr Meacher, follows similar complaints from the Irish government. It also follows a cull last month of seagulls and pigeons in the Sellafield area after their droppings were found to be radioactive.
Some of the heaviest concentrations found during investigations by the Norwegian Radiological Protection Board were in the claw and tail muscle of lobsters, parts which are usually eaten. However, much higher levels of radiation are allowed in British seafood.
The details were revealed in a written Parliamentary answer to Llew Smith, Labour MP for Blaenau Gwent, who has been campaigning on the issue.
Anna Lindh, chair of the Nordic Council of Environment Ministers and environment minister for Sweden, told Mr Meacher in a formal letter that the discharges were polluting some of the world's most valuable fishing grounds. Changes under consultation in Britain could result in even higher discharge levels from Sellafield, she said.
According to the Norwegian report, liquid discharges of a substance called Technetium-99 have increased to 50 times their 1994 level, while remaining well inside British safety levels. In the early 1990s, the waste was held back while a new processing plant was completed.
Ms Lindh's letter called for the discharges to be stopped. "The Nordic countries will closely follow the development of this issue," it said.
Mr Meacher replied that the discharges were not radiologically significant either to humans or to other species. The proposed changes would actually lead to a cut in the levels rather than an increase, he said.
"I share the concern of the Nordic countries for protection of the marine environment. While I appreciate that the detection of any levels of radioactivity can give rise to concern, and there is never room for complacency, it is important to consider the radiological impact. The UK is rigorous in adhering to all of the international commitments which it enters into in the field of environmental radioactivity," he said.