Iran: Imprisoned British Council staffer returns to UK after years in detention
Aras Amiri was detained by authorities in the Islamic Republic in 2018 and later jailed for 10 years in May 2019.
However, the British Council said she had been acquitted of charges against her following an appeal by her lawyer to Iran's Supreme Court.
"She has been freed from detention and has returned to the United Kingdom," the government-funded cultural organisation said in a statement on Wednesday, calling her ordeal "a long and difficult period".
The British Council, which describes itself as promoting culture and language in more than 100 countries across the world, has not had staff or offices in Iran since 2009.
The Iranian government in 2019 announced a ban on all collaboration with the organisation.
Amiri, who lives in the UK, was detained while on a trip to visit relatives in Iran. The British prime minister at the time, Theresa May, called her sentence "utterly shocking".
The UK government has condemned the detention of several Iranians with dual British nationality or based in the UK as hostage-taking aimed at pressuring the West.
Richard Ratcliffe has drawn parallels between Amiri's case and that of his wife, Nazanin Zaghari-Ratcliffe, who has been detained in Iran since 2016.
He wrote in The Guardian newspaper in 2019 that both were arrested while on holiday visiting family, targeted for their links to the UK, and had their jobs in London "turned into cynical espionage claims".
The pair had been cellmates in Evin prison, he added.
Ratcliffe last year held a three-week hunger strike outside the UK foreign office to encourage the government to reach a deal for her release.
His wife was sentenced to five years in prison for plotting to overthrow the Iranian government, then given another year's imprisonment in April this year for taking part in a rally outside the Iranian embassy in London in 2009.
Ratcliffe believes her detention - and those of other dual nationals - are linked to a UK debt of £400 million ($546 million) that London refuses to settle since the shah of Iran was ousted in 1979.