"The Foreign and Commonwealth Office (FCO) advise all British nationals against all travel to: within 100km of the entire Iran/Afghanistan border; within 10km [over 60 miles] of the entire Iran/Iraq border; the province of Sistan-Baluchistan; the area east of the line running from Bam to Jask, including Bam," the Foreign Office said in a statement.
The UK authorities also advised all British-Iranian dual nationals against all but essential travels to the rest of Iran over the risk to be arbitrarily detained.
"The Iranian authorities don’t recognize dual nationality for Iranian citizens and therefore don’t grant consular access for FCO officials to visit them in detention. If you’re a British-Iranian dual national and are subsequently detained in Iran, the FCO’s ability to provide consular support is extremely limited," the statement said.
The statement added that UK nationals should make visa application in advance as application's processing may require long time.