If the United Kingdom votes to leave the European Union on 23 June, the exit will break up cross-border collaborations and cut off E.U. funding for U.K. scientists. For fusion research, the possibility of a Brexit is particularly worry ing. Europe’s largest fusion facility, the Joint European Torus (JET), is sited just south of Oxford, U.K.; a vote to leave would put it in a legal limbo that could halt vital research supporting the International Thermonuclear Experimental Reactor (ITER), now under construction in southern France. JET dominates much of the work at its host institution, the Culham Centre for Fusion Energy. A Brexit “certainly will make us very vulnerable,” says Steve Cowley, the center’s director.

Polls suggest the referendum’s outcome is too close to call, and a vote for a Brexit would not affect some pan-European research facilities because the European Union does not control them. These include the CERN particle physics laboratory in Geneva, Switzerland; the Paris-based European Space Agency, which has a technology center at Culham; the European Southern Observatory in Garching, Germany; and the European Molecular Biology Laboratory, which has a bioinformatics institute in Cambridge, U.K.

Fusion is different. The nuclear arm of the European Union, known as Euratom, pays a consortium of national labs and university groups—dubbed EUROfusion—to carry out fusion research, most of which is aimed at supporting ITER. EUROfusion will receive €424 million for the work over 5 years (2014–18), and another €283 million will go to the Culham Centre for operating JET and hosting ITER-related research.

JET is the world’s largest tokamak—a doughnut-shaped fusion reactor like ITER. Its innards have been coated with the same beryllium and tungsten that will line ITER, and its heating power has been boosted to make it as ITER-like as possible. JET is also the only tokamak in the world equipped to use the fuel that will eventually power ITER: a mix of the hydrogen isotopes deuterium and tritium. Tritium is radioactive and hard to handle, but this mixture is the easiest to “burn,” meaning fusing its nuclei can release excess energy. JET researchers are currently testing how the machine behaves when filled with hydrogen, deuterium, and tritium individually before attempting deuterium-tritium burns in 2019.

A Brexit could halt those experiments. “We would be less well prepared for ITER operation,” says Lorne Horton, JET’s exploitation manager. “From a scientific point of view, I hope it doesn’t happen.” Tony Donné, head of EUROfusion in Garching, thinks that after a “leave” vote, the United Kingdom would operate the machine at least until the end of the current contract in 2018. But what might happen after that “is really crystal ball–gazing.” The United Kingdom could opt to buy into the Euratom research program, as some non-E.U. nations like Switzerland have done, but that process could take years and is likely far down the list of negotiations that the U.K. government will need to complete following a Brexit vote.

Donné says EUROfusion has been working on plans to turn JET into a fully international facility so that the other ITER partners—China, India, Japan, Russia, South Korea, and the United States—can also use it for ITER preparation. “If it flies, this would be a fantastic project,” Donné says. By cutting ties to Euratom, a Brexit could scuttle that plan, too.