Cantons Basel City and Jura have voiced their concerns about a French nuclear facility 35 kilometres from the Swiss border.
They fear earthquake risks have not been properly taken into account at France's oldest nuclear plant, which began operations in 1977. The site at Fessenheim is considered to be in a seismic-risk zone.
The two cantons mandated a private firm to evaluate the potential danger after the French and Swiss authorities did not respond to repeated requests for information. The specialists came to the conclusion that the seismic threat had been underestimated in the past.
The biggest earthquake ever to hit Switzerland struck Basel in 1356, destroying much of the city.
More recently, a series of mini-tremors was set off by a controversial geothermal project, causing minor damage to buildings in the Basel area.
Earlier this year, a Japanese nuclear facility suffered major damage after an earthquake struck nearby, and later analysis showed that it was not capable of resisting a big tremor.