French researcher Laurent Vinatier, who is already serving time in a Russian prison, is now facing espionage charges that could extend his sentence by up to 20 years.
Vinatier is to appear at a hearing on Monday to face new charges of espionage, which carry a sentence of up to 20 years in prison, according to court documents reported by Russian news agencies.
Vinatier, who worked for a Swiss conflict mediation organisation, is currently serving a three-year prison sentence after he was convicted in October of violating Russian laws requiring individuals deemed "foreign agents" to register, and was accused of gathering military information of value to foreign intelligence services.
His appeal against the sentence was rejected in February.
The 49-year-old is one of several Westerners to have been charged under Russian security laws at a time of tense confrontation between Moscow and the West over the war in Ukraine.
An expert on the former USSR, Vinatier told the court at his trial that he loved Russia and apologised for breaking the law.
He said he always tried to "present Russia's interests in international relations".
France said Vinatier had been arbitrarily detained and called for his release.
French President Emmanuel Macron denied that Vinatier worked for the French state and has described his arrest as part of a misinformation campaign by Moscow.
(with newswires)
