The Foreign Office states: "Recent media reports suggest Iran is willing to detain people who have been visiting relatives inside Iran.
Iranian state media said on Wednesday that a man and woman were in custody in Iran's southeastern city of Kerman on security-related charges.
A spokesperson said: "We are providing consular assistance to two British Nationals detained in Iran and are in contact with the local authorities.".
The Foreign Office warn that people are at risk of "arrest, detention and a death sentence" if they travel to Iran.
The department says that Tehran does not recognise dual nationality, and people with Iranian citizenship will be considered Iranian.
