Britons are no longer being warned against all travel to Iran in the latest sign of a thaw in relations.
The Foreign Office had previously judged there was an “unacceptably high” risk to those visiting the country.
But while it is still urging UK nationals not to go to the areas bordering Iraq, Afghanistan and Pakistan, it has downgraded guidance to avoid travel elsewhere unless it is “essential”.
Foreign Secretary Philip Hammond said the move was in part due to the constructive approach adopted by President Hassan Rouhani.
“Travel advice for every country is reviewed on a regular basis and informed by the latest information,” he said.
“Our policy is to recommend against travel to an area when we judge that the risk is unacceptably high.
“We consider that continues to be the case for specific areas of Iran, notably along Iran’s borders with Iraq, Afghanistan and Pakistan.
“But we believe that in other areas of Iran the risk to British nationals has changed, in part due to decreasing hostility under President Rouhani's Government”.
The Foreign Office said Britons should still “consider carefully the risks of travelling to Iran”.
The UK Embassy in Tehran remains closed for the time being, and travellers needing urgent help would have to contact the Swedish or EU diplomatic missions.
The change in advice comes after Iran struck a deal with world powers over its nuclear programme, with some sanctions being lifted in return for allowing inspections.