Britain and Iran have officially restored diplomatic ties, which were severed by London after a group of Iranian hard-liners stormed its Tehran embassy in 2011.
"The UK has agreed with Iran that from today [February 20] bilateral relations will be conducted directly through non-resident charge d'affaires and officials," a Foreign Office spokesman was quoted as saying.
Iranian Deputy Foreign Minister Majid Takht-Ravanchi also confirmed to the ISNA news agency that the two countries had re-established direct diplomatic ties.
Relations between the two countries have improved since relative moderate President Hassan Rohani took power in August 2013.
Britain's new envoy, Ajay Sharma, visited Iran in December.