The British embassy in Iran has reopened, nearly four years after it was closed.
Foreign Secretary Philip Hammond attended a ceremony in Tehran with Iranian diplomats to mark the reopening while Iran simultaneously reopened its embassy in London.
The UK embassy was closed in 2011 after it was stormed by protesters during a demonstration against sanctions.
Mr Hammond is the first UK foreign secretary to visit Iran since 2003.
The visit comes weeks after Iran reached a deal with six world powers aimed at curbing its nuclear programme.
At the ceremony Mr Hammond said the attack in 2011 had been "a low point" but since the election of President Hassan Rouhani things had "steadily improved, step by step".
He said: "Last month's historic nuclear agreement was another milestone, and showed the power of diplomacy, conducted in an atmosphere of mutual respect, to solve shared challenges.
"Re-opening the embassy is the logical next step. To build confidence and trust between two great nations.
"Iran is, and will remain, an important country in a strategically important but volatile region. Maintaining dialogue around the world, even under difficult conditions, is critical.
"We will not always agree. But as confidence and trust grows there should be no limit to what over time we can achieve together and no limit to our ability to discuss these issues together."
Mr Hammond and new head of the embassy Ajay Sharma watched as the Union flag was raised
Representatives of the Iranian Ministry of Foreign Affairs and the wider diplomatic community are also in attendance.
Initially, the embassy will be headed by a charge d'affaires, Ajay Sharma, but Mr Hammond said an agreement on upgrading to full ambassador status is expected to be reached in the coming months.
On arriving in Tehran, Mr Hammond tweeted: "First British Ministerial visit since 2003. Historic moment in UK-Iran relations."
A trade delegation has travelled to Tehran with Mr Hammond and the Exchequer Secretary to the Treasury Damian Hinds to discuss possible future trade opportunities.
In November 2011 Iran announced it was expelling the UK's ambassador in retaliation for British support for tougher sanctions on Tehran over its nuclear programme.
Hundreds of protesters stormed embassy compounds two days later, smashing windows, torching cars and burning Union flags.
The UK responded by closing the Iranian embassy in London later that month.
But following the election of Hassan Rouhani and an agreement on how to deal with Iran's nuclear programme, the then Foreign Secretary William Hague proposed the reopening of the embassy in June last year.
Since then, the reopening of the embassy has been held up by technical problems over visa policy and communications equipment, Mr Hammond has said.