More than 300,000 tonnes of “radioactive” mud, some of it the toxic byproduct of Britain’s atomic weapons programme, will be dredged to make way for England’s newest nuclear power station and dumped in the Severn estuary just over a mile from Cardiff.

Politicians in Wales have denounced the move, with one accusing the Welsh government of selling out to London and the nuclear lobby. They have called on ministers to commit to further radiological tests before giving consent for the process, which is crucial for the construction of Hinkley Point C across the estuary in Somerset.

In 2013, an assessment by the Centre for Environment Fisheries and Aquaculture Sciences found that tests on the mud, carried out under International Atomic Energy Agency procedures, revealed only minimal levels of radioactivity, well within safety guidelines. “From radiological considerations, there is no objection to this material being dredged and dumped,” the assessment concluded.

But John Thomas, leader of the Vale of Glamorgan Council, said concerns had been raised about the adequacy of the assessment. He has called for an urgent meeting with Natural Resources Wales, which approved the dredging licence, and the Welsh government.

An independent marine pollution researcher, Tim Deere-Jones, who is also a prominent nuclear power critic, has warned that the dumped sediment could re-concentrate into more powerful radioactive material and be washed ashore in storm surges. “We know sediment in mudflats can dry out and blow ashore and that fine sediment with radioactivity attached can transfer to the land in marine aerosols and sea spray,” Deere-Jones said. Studies of north Wales tidal surges, he added, had revealed that the deposited mud and sand were heavily contaminated with radioactivity from Sellafield.

The mud to be dredged contains 50-year-old deposits from Hinkley Point A, where radioactive material for Britain’s atomic weapons was produced. Nuclear historian Dr David Lowry said some of the plutonium produced at the plant was sent to the US in a controversial and confidential exchange. “That deal is coming back to haunt today’s nuclear industry as plans for the third generation of nuclear plants at Hinkley are literally running into the sparkling radioactive mud,” Lowry said.

The dredging licence was granted to EDF, the company building the new plant, in 2013. It gives the French energy giant the right to discharge materials at Cardiff Grounds, a sandbank in the Bristol Channel. EDF needs to clear the sediment to allow barges to bring in construction materials.

Neil McEvoy, Plaid Cymru Welsh Assembly member, who has received a briefing from Deere-Jones, said dredging should be suspended until a full environmental impact assessment had been carried out. A petition has gained enough signatures to force the assembly to debate the issue. “The problem lies with the unknown,” McEvoy said. “No dose of non-naturally occurring radiation is safe. What we have here is big business trampling over Wales, with a Welsh government doffing its cap to London and the nuclear industry,” he said. “The Welsh national interest is not being served here. The public is outraged that this material will be dumped in Cardiff’s waters, and washed around the Welsh coast, with transfer of radioactivity from sea to land.”

An EDF spokesman said: “We have undertaken a number of assessments as part of this application which concluded the activities pose no threat to human health or the environment. All activities on our sites are strictly controlled and regulated by a number of statutory bodies to ensure the environment and public are protected.”

John Wheadon, of Natural Resources Wales, said: “There are strict conditions in the licence to test the sediment, including a radiological assessment, before it can be deposited. We will only allow the work to start if we’re confident the activity will not harm people and the environment.”