Britons with dual Iranian nationality have been told to avoid all but essential travel to Iran because of an increased risk they will be “arbitrarily detained” by its hardline Islamic regime.
The Foreign Office said Jeremy Hunt had taken the decision to heighten the warning against unnecessary visits to the Middle Eastern country because of the risks “we have seen all too sadly in a number of cases”.
Tehran continues to hold a British-Iranian charity worker, Nazanin Zaghari-Ratcliffe, in prison after convicting her of spying, which she denies.
Mr Hunt has previously vowed to leave “no stone unturned” in his efforts to secure her release.
An FCO spokeswoman said: "The Foreign Secretary has taken the decision to advise against all but essential travel by UK-Iranian dual nationals to Iran.
"British citizens who also hold Iranian nationality face risks if they travel to Iran, as we have seen all too sadly in a number of cases.
"The Iranian government does not recognise dual nationality so if a dual national is detained our ability to provide support is extremely limited.”
Mrs Zaghari-Ratcliffe, from Hampstead, north London, was sentenced to five years in jail after being accused of spying by Tehran’s Islamist regime.
She denies the allegation and said she was on holiday in Iran to allow her daughter to spend time with relatives there.
Her four-year-old daughter Gabriella has been staying with family since Mrs Zaghari-Ratcliffe, who works for the Thomson Reuters Foundation, was detained at Imam Khomeini airport in April 2016.
The new travel advice warns: "British nationals, in particular dual British/Iranian nationals, face greater risks than nationals of many other countries.
"The security forces may be suspicious of people with British connections.
"The risks are likely to be higher for independent travellers or students than for people travelling as part of an organised tour or business people invited by the Iranian authorities or companies.
"If you have links to any organisation perceived as being anti-Iranian, either within Iran or elsewhere, you may be at even greater risk."
It goes on to warn that if people are detained the UK Government has "serious concerns that the subsequent judicial process falls below international standards".
Iran has been rocked by economic unrest in recent months and US sanctions linked to its nuclear programme due to be reinstated in November are likely to make matters worse.
The travel advice continues: "Any behaviour that doesn’t have an obvious explanation can put you at risk, no matter how innocent you believe it to be.
"This may include travel off the beaten track, being present near crowds or sensitive sites, having contact with Iranians who are of interest to the authorities, taking photographs (except in major tourist sites), or behaviour that could be perceived as contrary to official Iranian interpretations of Islam.
"The threat to travellers is likely to be higher if there’s any national unrest, terrorist incident or an increase in tensions between Iran and the international community."