European research ministers say they are ready to press ahead with the Iter nuclear fusion project even if it means losing Japanese support.
The multi-billion-euro reactor will produce energy from nuclear reactions like the ones that fuel the Sun.
But the international project has been stalled because the parties involved cannot agree on a location to build it.
Now, EU ministers say that if no deal is done soon, they will go it alone with a reactor at Cadarache in France.
" The fact that they are setting a deadline for their rival to make a concession is something like a declaration of war "
He added: "If the negotiations do not come to a rapid conclusion, the commission has the possibility to choose a different path."
And the EU's research commissioner, Janez Potocnik, added: "I do not think that it is useful to put a deadline on this. We have not discussed that today.
"As long as there is the slightest possibility of a positive outcome, I am committed to negotiate."
Different views
After the International Space Station, the International Thermonuclear Experimental Reactor (Iter) would be the largest global research and development collaboration.
There are currently six parties involved. The EU has the support of China and Russia to build the reactor at Cadarache.
Japan has the backing of the US and South Korea to construct Iter at Rokkasho in the north of its territory.
A decision on the location should have been made a year ago - but the parties are deadlocked.
Europe would like Japan to stand down and accept a major support role - in what the EU calls a "genuine partnership". This would involve Japan getting a materials testing facility needed to push forward the Iter project.
The facility would also be well placed to win any contracts to supply the commercial reactors that could come after Iter.
Japan, however, is adamant that it has the best candidate site and has been upset by the EU's attempt to force the issue by threatening to go it alone.
"It is extremely regrettable. We hope that the EU will handle this matter appropriately and honestly," said Takahiro Hayashi, deputy director of the Office of Fusion Energy at Japan's energy ministry.
"There is no deadline for the talks. We will continue until both sides reach an agreement," he told the AFP news agency.
And the office's director, Satoru Ohtake, told Reuters news agency: "The two sides have different ideas, and therefore we should take time to have good discussions."
He added: "The fact that they are setting a deadline for their rival to make a concession is something like a declaration of war."
Technical obstacles
Unlike in fission reactions, in which atomic nuclei are split to release energy, fusion reactions release energy when nuclei are forced together.
The process is the same as the one that powers the Sun. Achieving stable and sustained reactions on Earth, however, present an immense challenge.
The Iter design is for the reactions to take place inside a 100-million-degree gas (plasma) suspended in an intense doughnut-shaped magnetic field.
Several research facilities, such as the Joint European Torus (Jet) project at Culham, UK, have shown this is feasible; but none has so far been able to sustain the reactions for long periods.
Iter will consolidate all that has been learnt over many decades of study. It is expected to produce 500MW of fusion power during pulses of at least 400 seconds.
If it achieves this and its technologies are proven to be practical, the international community would then build a prototype commercial reactor, dubbed Demo.
Fusion could help fill the void as the world moves away from oil, coal and natural gas.
The fusion fuels are plentiful and produce no greenhouse emissions when "burnt". The systems are said to be inherently safe because they shutdown in a malfunction; and although radioactive materials are produced, they are not of the high-level long-lived variety that has so burdened nuclear fission.
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