France has described spying charges brought against one of its citizens in Iran as incomprehensible and said it knew of nothing likely to back up the claims.
The French foreign ministry said Benjamin Briere was on holiday in the country when he was arrested in May 2020.
Accused of flying a drone and taking photographs in a prohibited area, Mr Briere was charged with espionage and "propaganda against the system".
He is one of a number of foreign nationals to have faced charges in Iran in what rights groups condemn as a policy of hostage-taking to win concessions from foreign powers.
British-Iranian citizen Nazanin Zaghari-Ratcliffe, who was arrested in Iran in 2016, was handed an additional one-year jail sentence in April in a move linked by her husband to ongoing talks over the Iran nuclear deal in Vienna.
Mr Briere’s lawyer Saeed Dehghan said the Frenchman was awaiting court proceedings in Iran.
"The prosecutor is preparing the indictment and sending it to the revolutionary court for the continuation of the judicial process," Mr Dehghan told AFP.
Espionage is punishable by death in Iran, while the charge of propaganda against the system can lead to a prison sentence of three months to one year.
Other accusations against Mr Briere, including the potential charge of “corruption on earth”, were dropped after an investigation.
Iran open to prisoner swaps as nuclear talks continue
The latest developments come days after Mr Briere’s sister appealed to French President Emmanuel Macron to push for her brother’s release.
Blandine Briere said in an open letter that the charges against her brother were baseless and that he had become a negotiating tool.
Iran's negotiators have said during the Vienna talks that Tehran is open to exchanging prisoners.
World powers have been engaged in negotiations with Iran since April in an effort to revive the deal which Donald Trump abandoned in 2018.
Over the past two years, Iran has conducted several exchanges of foreign prisoners with countries holding Iranian nationals.
Mr Briere is a rare case among such detainees as he does not also have an Iranian passport.
Tehran, which does not recognise dual nationality, insists that all those held have been subject to proper judicial process.
The goal of the Vienna talks is to return Washington to the pact and lift sanctions on Tehran, while also bringing Iran back into compliance with limits on its nuclear activity.
These are aimed at preventing Iran from developing a nuclear weapon. Iran maintains it has no such intention.
Negotiations in the Austrian capital are in their fifth round with no plans for a sixth, Russia's envoy to the talks said.
"The negotiators proceed from the understanding that the current round should be final," Mikhail Ulyanov said on Sunday.