Britain is on course to break an international agreement to reduce radioactive pollution of the seas, because of an increase in activity at the Sellafield nuclear site, according to a report from a campaign group that monitors the plant.

Discharges of radioactive waste into the Irish Sea from the nuclear fuel reprocessing plants at Sellafield, Cumbria, are set to double over the next few years because of a “crash programme” of reprocessing planned by the government’s Nuclear Decommissioning Authority (NDA).

Critics say this would put the government in breach of its commitment to “progressive and substantial reductions of discharges” under the Oslo-Paris (Ospar) convention, which seeks to limit pollution of the north-east Atlantic. The convention’s agreed aim is to bring levels of artificial radioactivity in the environment down to “close to zero” by 2020.

Breaching the convention, which brings together 15 governments from across Europe, would be politically embarrassing for Britain and could expose ministers to legal action from other countries or environmental groups. Ospar’s radioactive substances committee is due to meet on Monday in Monaco.

The report, by anti-nuclear group Cumbrians Opposed to a Radioactive Environment (Core), estimates that discharge of plutonium into the sea from Sellafield will rise from 120 gigabecquerels a year to more than 250. There will be similar increases in the levels of radioactive isotopes caesium-137 and cobalt-60 compared with the past five years, it says.

Core’s spokesman, Martin Forwood, accused the NDA of “breathtaking complacency” and demanded an end to reprocessing. “Officialdom is sleepwalking towards a situation which, unless avoiding action is taken now, will see commitments broken and a further trashing of the marine environment courtesy of Sellafield reprocessing,” he said.

When Britain signed up to Ospar commitments on radioactive discharges in Portugal in 1998, the then deputy prime minister, John Prescott, said that it had shed the tag of “dirty old man of Europe”. Now the country will regain the label, claimed Forwood.

The NDA, however, insisted that it remains committed to “full compliance” with the Ospar convention. “Sellafield discharges are well within authorisations and doses from discharges are very much below the legal limit,” said an NDA spokesman.

“Increases in productivity will not, and cannot, be to the detriment of the environment. As such it is incorrect to suggest that the NDA will in any way prejudice our commitments to Ospar.”

But when questioned at a stakeholder meeting last year, the NDA admitted that it needed a “contingency plan” if it failed to meet Ospar obligations. One option was “agree not to meet OSPAR deadline”, it said.