A British parliamentary delegation led by former foreign secretary Jack Straw met Iranian Foreign Minister Javad Zarif on Tuesday at the start of the first such visit in years, Iranian state television reported.
Long-strained ties between the two countries have warmed since the relatively moderate Hassan Rouhani was elected Iran's president in June.
In December, a new non-resident charge d'affaires made Britain's first diplomatic visit to Iran since London downgraded ties after hardliners stormed its embassy in 2011. The two countries' respective embassies remain closed, however.
Iranian media said the delegation would also meet parliamentary speaker Ali Larijani and the head of the parliament's National Security and Foreign Policy Committee, Alaeddin Boroujerdi.
Rouhani's election has also signaled detente on the thorny issue of Iran's sensitive nuclear program; six world powers including Britain agreed an outline deal in November that will curb the program in return for an easing of economic sanctions.
The other members of the British delegation are former finance minister Norman Lamont, Conservative member of parliament Ben Wallace and Labour lawmaker Jeremy Corbyn.