Britain has upgraded its travel warning for dual UK-Iranian nationals after a British council worker was jailed on allegations of espionage.
The Foreign Office on Friday warned dual nationals against all travel to the Islamic Republic, saying they face an "unacceptably higher" risk of detention and mistreatment. Its previous advice had warned against all but essential travel.
In a statement, Jeremy Hunt, the Foreign Secretary, also urged Iranian nationals living in the UK to exercise "caution" when returning to Iran to visit family or friends.
"Despite the UK providing repeated opportunities to resolve this issue, the Iranian regime's conduct has worsened," Mr Hunt said.
"Having exhausted all other options, I must now advise all British-Iranian dual nationals against travelling to Iran."
He said Iran does not recognise dual nationality, limiting the British government's ability to help dual nationals detained in Iran.
The decision comes days after it was revealed a dual national who had been working for the British Council was sentenced to 10 years in prison on allegations of spying.
Aras Amiri, from London, is being held in Evin prison alongside 40-year-old charity worker Nazanin Zaghari-Ratcliffe, who was offered official diplomatic protection by the British government in an unprecedented move to secure her release.
Richard Ratcliffe, Ms Zaghari-Ratcliffe's husband, said the two women were both chess pieces on the same political board.
Reacting to the Friday's news, Mr Ratcliffe told the Telegraph: "I am pleased the government has made such a strong statement. Hopefully the Iranian authorities will realise this practice has to stop.
"I asked the Foreign Secretary to make clear hostage diplomacy is not acceptable - through travel advice and other ways. The UK has an obligation to protect, and to make clear it is not acceptable for government disputes to be taken out on ordinary citizens."
Hamid Baeidinejad, Iran's ambassador to the UK, said dual nationals were perfectly safe provided there were not working for foreign intelligence agencies.
The past week has seen an escalation in tensions between Iran and the US. Concerns about a possible conflict have flared since the White House ordered warships and bombers to the region to counter an alleged threat from Iran that has seen America order nonessential diplomatic staff out of Iraq.
Tensions have also ratcheted up in the region after authorities alleged that a sabotage operation targeted four oil tankers on Sunday off the coast of the United Arab Emirates, and Iran-aligned rebels in Yemen claimed responsibility for a drone attack Tuesday on a crucial Saudi oil pipeline.
Mr Hunt said this week that the United Kingdom shares the US assessment of increased threat, but that British diplomatic missions in Iraq continued to operate as normal.