Two people with links to the UK have been arrested in Iran, according to the Foreign Office and the British Council.
Mahan Abedin has dual Iranian and British citizenship - while Aras Amiri is an Iranian living in London who works for the British Council.
The Foreign Office said it is "urgently seeking information" about Mr Abedin.
He is not the first UK citizen to be detained in Iran recently - Abbas Edalat, a British-Iranian academic, was arrested in Tehran in April.
And Nazanin Zaghari-Ratcliffe, a British-Iranian arrested in April 2016, is serving a five-year jail sentence in Iran.
The British Council said Ms Amiri was not working in Iran - the group, which aims to improve cultural relations, was stopped from working in the country by then-President Mahmoud Ahmadinejad in 2009.
The council said she was on a private trip to visit family.
A spokeswoman said: "We are aware that one of our staff has been detained in Iran while making a private family visit.
"The colleague is an Iranian national. Despite an assertion that this individual has travelled to Iran for work this is not the case."
Ms Zaghari-Ratcliffe was detained while on what her family describe as a holiday with her daughter.
Her husband, Richard Ratcliffe, has led a campaign urging the British government to secure her release. Their daughter remains in Iran with her grandparents, and Mr Ratcliffe has not been able to see her.
Ms Zaghari-Ratcliffe was sentenced for five years on charges of seeking to overthrow the Iranian government.
She remains in prison despite an Iranian diplomat insisting the government was doing its best to secure her release.
In November, Reuters reported that Iran had arrested at least 30 dual nationals - mostly Europeans - in the previous two years. Most were accused of spying.