Britain lifted an official warning against all but essential travel to Iran on Saturday, citing "decreased hostility" in the wake of a landmark nuclear deal.
Almost all of Iran is now marked "green" in the government's foreign travel advice, though border areas with Iraq, Pakistan and Afghanistan remain "red", a warning against all travel.
In a statement, the Foreign and Commonwealth Office said that British nationals should still carefully consider the risks of travelling to Iran, but that the situation had changed under the government of President Hassan Rouhani.
"Our policy is to recommend against travel to an area when we judge that the risk is unacceptably high," Foreign Secretary Philip Hammond said in a statement.
"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."
Britain, China, France, Russia, the United States and Germany reached a long-elusive agreement this month to limit Iran's nuclear activities in exchange for lifting sanctions that have crippled the country's economy.
The British government has said it hopes to reopen its embassy in Tehran by the end of this year. It was closed in 2011 after being stormed by protesters.