British embassy in Iran capital Tehran reopened nearly four year after it was closed.
Foreign Secretary Philip Hammond is attending a ceremony in Tehran with Iranian diplomats to mark the reopening.
Iran is simultaneously reopening its embassy in London.
The UK embassy was closed in 2011 after it was stormed by protesters during a demonstration against sanctions.
Philip Hammond is the first British 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.
