British-Iranian dual nationals have been warned by the UK Foreign Office against all but essential travel to Iran.
A spokesperson said Britain’s ability to provide support to detained dual nationals in Iran was “extremely limited” as the country does not recognise their status.
It comes as the British-Iranian charity worker Nazanin Zaghari-Ratcliffe is detained in Tehran, convicted of spying.
She denies the charges, and human rights groups have called for her release.
Sky News understands the significant travel advice announcement came in a personal intervention by Jeremy Hunt, the foreign secretary.
Mr Hunt is said to feel responsible for British-Iranian dual nationals in Iran and thought a tougher approach is required.
He has been considering the move since he started the job in July, and believed that the previous advice was not right and the UK needed to be honest.
It is the first time the Foreign Office has given travel advice specifically to dual nationals.