Benjamin Brière, 36, was arrested after flying a drone near the Iran-Turkmenistan border.

If convicted he could receive a jail term, though his Iran-based lawyer has implied authorities are considering including him in a prisoner exchange.

Mr Brière has been on hunger strike since December, leaving him very weak, another of his lawyers has said.

Philippe Valent, Mr Brière's lawyer in Paris, and Mr Brière's sister said a verdict might be issued on Saturday. He is being tried behind closed doors in the Revolutionary Court in the eastern city of Mashhad, where he is being held.

His Iranian lawyer, Saeed Dehghan, tweeted in Persian that the court had upheld charges of espionage and of "propagating against the system".

He also suggested the court might be planning to use Mr Brière in a prisoner swap, tweeting: "Using the word #exchange, they made their stance clear."

Earlier this week Philippe Valent said that his client had not been informed of the charges against him.

Mr Brière's sister Blandine told AFP news agency that the family "have very little information, all we know is that the hearing took place and that a verdict will be issued on Saturday".

His family says he is innocent and is being used as a political pawn.

Mr Brière was arrested in May 2020 while on holiday in Iran, the French foreign ministry has said. He is one of more than a dozen foreign nationals or people with dual Iranian citizenship held in Iran, though he is the only known Western detainee who does not also hold an Iranian passport.

His case comes at a time of heightened tensions between Iran and Western powers, at loggerheads over Iran's nuclear programme.

Iran and France, Germany and the UK have been holding talks in Vienna for several months aimed at restoring an international deal to curb Iran's nuclear activities in exchange for the lifting of economic sanctions, an agreement which unravelled after the US withdrew its support in 2018.

So far the talks have been inconclusive, with each side warning that the negotiations are running out of time.

Foreign powers say they are not convinced by Iran's assertion that its nuclear programme is for purely peaceful purposes and not for building a bomb.