An effigy of Britain's ambassador to Iran was set alight in Tehran as the Iranian judiciary declared the envoy an "undesirable element".
Over the weekend Rob Macaire was accused of attending an "illegal" protest despite his denials. The ambassador said he had attended a vigil for victims of the devastating plane crash over Iran. London said his detention was a violation of diplomatic conventions.
His short detention sparked outrage in Britain and elsewhere over the treatment of a diplomatic figure.
British Foreign Minister Dominic Raab said London had received no formal notification that Mr Macaire would be expelled, adding that any such action would be regrettable.
"Based on international regulations, the British ambassador in Iran is an undesirable element," judiciary spokesman Gholamhossein Esmaili told reporters.
Prominent hardline Iranian cleric Ayatollah Ahmad Alamolhoda also said expelling the ambassador would be "the best thing that can happen to him" as otherwise loyal supporters of assassinated Quds Force general Qassem Suleimani would "chop him to small pieces".