Britain is threatening to return boatloads of radioactive waste to Europe if an agreement on post-Brexit nuclear regulation is not reached.

The UK has 126 tonnes (139 US tons) of plutonium at Sellafield nuclear plant – the world’s largest civilian stockpile. Almost a fifth of the material originates from other countries including Italy, Germany and Sweden.

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The state-owned plant in Cumbria has been reprocessing spent nuclear fuel from across Europe since the 1970s, producing reusable uranium, plutonium and radioactive waste.

In what is being taken in Brussels as a thinly-veiled threat, a paper setting out the UK’s position for the negotiations stresses the right “to return radioactive waste … to its country of origin” should talks collapse.

The Department for Exiting the European Union’s paper raises the complex question of what should happen to the nuclear waste once Britain leaves the Euratom treaty, which established the European Atomic Energy Community and regulates the nuclear industry across Europe.

Nuclear experts who have advised the government told the Financial Times that the UK’s warning over the future ownership of radioactive waste might just encourage a more flexible approach from Europeans over the issue.

“It might just be a reminder that a boatload of plutonium could end up at harbor in Antwerp [in Belgium] unless an arrangement is made,” one source told the newspaper.

EU diplomats hit back at the threat by telling the paper they would have “the coastguard ready.”

The paper also highlighted the responsibility of EU countries for some “special fissile materials” – the most dangerous and tightly-regulated types of nuclear substances, including plutonium – derived from imported spent fuel.

Britain has said while it is leaving the Euratom treaty, of which it has been a member since 1957, it wants to continue to cooperate on nuclear regulation after the UK leaves the union in March 2019.

Nuclear regulation has become one of the most contentious issues in the early stages of Brexit negotiations, as Britain must disentangle itself from the Euratom agreement.

Leaders of the UK nuclear industry are lobbying the government to find a way of remaining part of Euratom, or, if that proves impossible, to negotiate an extended transition deal to allow time to establish a new regulatory system.

Those arguing for Prime Minister Theresa May to compromise have highlighted that the failure to secure a nuclear deal with the EU could disrupt UK supplies of nuclear reactor parts, fuel and medical isotopes vital for the treatment of cancer.

Around 500,000 scans are performed in the UK every year using imported radioisotopes.