UK must tread carefully in new deal with Iran: British Foreign Secretary

"We should tread carefully, there's a deep legacy of distrust on both sides and we have major areas where have very substantial policy differences.

"So we should tread cautiously, but that doesn't mean we shouldn't be talking, that we shouldn't attempt dialogue, that we shouldn't discuss areas where we do see eye-to-eye.

"We do see eye-to-eye on the need to interdict the opium traffic between Afghanistan and Europe, we do see eye-to-eye on the need to challenge Isil, for example, and there are areas where we can work together.

And as we work more closely together in those areas we will find it easier to have a sensible dialogue about the many areas on which we disagree, including human rights in Iran."

Speaking ahead of a meeting with Mr Rouhani, Mr Hammond added: "Iran is too large a player, too important a player in this region, to simply leave in isolation without any interaction with them."

Plans to reopen the embassy were announced by the Government last year as relations between London and Tehran improved under president Rouhani and Iran's strategic position came under the international spotlight as the crisis sparked by IS militants grew in neighbouring Iraq.

Last month, Iran struck a deal after a decade of negotiations with world powers over its nuclear programme, with some sanctions being lifted in return for allowing inspections.

The Foreign Office later eased its advice against travelling to Iran, saying the risk faced by British nationals had changed in parts of the country.

Mr Hammond is the first British foreign secretary to visit Tehran since 2003 and only the third UK minister to visit since the 1979 revolution.