A British-Iranian professor who had been held in Iran since April has returned to England.
Abbas Edalat, who teaches mathematics and computer science at Imperial College London, was arrested and held on unspecified security charges.
Edalat, who has dual Iranian and British citizenship was in Iran to visit family and attend an academic conference at the time of his arrest.
The Campaign Against Sanctions and Military Intervention in Iran, charity Edalat founded, said he was released last week.
"It increasingly appears, as we had suspected, that his detention in spring in Iran was a case of misinformation and misunderstanding by the Iranian security apparatus," the group said in a statement.
Edalat was one of dozens of dual citizens in Iranian jails, including Nazanin Zaghari-Ratcliffe who has been jailed since 2016.
The British-Iranian charity worker is serving a five-year sentence for spying, which she denies.
On Tuesday, British Foreign Secretary Jeremy Hunt tweeted, "As we celebrate, let's also remember those unjustly imprisoned like Nazanin Zaghari-Ratcliffe who is 40 tomorrow."
Happy Christmas to all. As we celebrate, let%27s also remember those unjustly imprisoned like Nazanin Zaghari-Ratcliffe who is 40 tomorrow and separated from her brave husband Richard and devoted daughter Gabriella. Many more like her - we will not forget them today #FreeNazanin— Jeremy Hunt (@Jeremy_Hunt) December 25, 2018
Iran, which does not recognize dual citizenship, has been accused of arresting and jailing Iranians with second citizenships in order to use them as win concessions in negotiations.