A Swedish citizen working for the EU diplomatic corps has been imprisoned in Iran for more than 500 days, making him an important bargaining chip for Tehran as it tries to wring concessions from the West.

The arrest, which has been kept under wraps for over a year by Swedish and EU authorities, appears to be part of an expanding pattern of what has become known as Iran’s “hostage diplomacy”.

Tehran has been opportunistically scooping up dual Iranian nationals and foreigners on spurious charges, seeking to trade them for Iranians held in Europe or the US.

Last month the US concluded a deal with Iran to free five Americans held there in exchange for $6 billion in withheld Iranian oil revenues as well as the release of Iranian prisoners in America.

Still, this latest case, the details of which have not been previously reported, stands out for the prisoner’s professional background as a European official. The man, Johan Floderus, 33, a native of Sweden, has held several positions in the EU’s institutions, coming up through its civil service traineeship programme.

He was even featured in an advertising campaign to attract young Swedes to EU careers.