European Commission research spokesman Fabio Fabbi said the EU hoped a deal would be clinched in Vienna on Tuesday to build the International Thermonuclear Experimental Reactor (ITER) at Cadarache, near Marseille, rather than at a rival site in Japan.
The reactor's costs are estimated at $12 billion.
"Our priority is to get an agreement with the largest number of participants and if possible with all six partners."
The partners would be the EU, the United States, Russia, China, Japan and South Korea.
"If there is no agreement, we'll have to think over how we go ahead with a maximum number of partners who want to participate," he said.
EU research and industry ministers are due to discuss how to move forward at a meeting on 25-26 November and the commission will recommend a course of action depending on the outcome of the Vienna talks, Fabbi said.
An EU source on Monday said that Cadarache was set to win the contest because Japan had signalled it would drop its bid in return for compensation.
The EU is pushing to build the
reactor at Cadarache
But an official at the Japanese Science and Technology Ministry said Tokyo had not ended its bid to host the project.
Nuclear fusion has been touted as a long-term solution to the world's energy problems, as it produces low pollution and uses limitless sea water as fuel. The idea is to replicate the way the sun generates energy.
Fusion involves fusing atoms together, as opposed to today's nuclear reactors and weapons, which produce energy by splitting atoms apart.
However, 50 years of research have failed to produce a commercially viable fusion reactor.