The most senior British diplomat to visit Iran since the sacking of Britain's embassy in 2011 held talks in Tehran on Monday with Iranian officials.
Political Director Simon Gass would discuss a "range of bilateral and international issues" with Iranian officials, the Foreign Office in London said.
Gass is also Britain's lead negotiator in talks on Iran's nuclear programme and was Britain's ambassador in Tehran between 2009 and 2011.
Ties between Britain and Iran have recently warmed, with newly-named non-resident charges d'affaires making a series of trips between Tehran and London.
"Sir Simon's visit is the next stage in the step-by-step approach to improving relations between our two countries," a Foreign Office statement said.
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"During his visit, Sir Simon is holding discussions with his Iranian counterparts on a range of bilateral and international issues."
According to a Tehran-based Western diplomat aware of the trip, the visit is about reopening Britain’s embassy in Tehran, not upcoming talks on Iran's disputed nuclear programme.
But Iran's state-run IRNA news agency said Gass had discussed "international issues including the nuclear negotiations" when he met with deputy foreign ministers Abbas Araqchi -- who is also a senior nuclear negotiator -- and Majid Takht-Ravanchi.
The discussions also focused on "the situation of the embassies, consular services and employees," IRNA said.
Britain shuttered the embassy when hundreds of Islamist students stormed the compound in November 2011, and then ordered the closure of Iran's mission in London.
The students -- protesting against Western sanctions over Tehran's nuclear programme -- ransacked the building as well as the ambassador's residence in north Tehran.
Iran and Britain officially resumed diplomatic relations in February.