LONDON, JUNE 18 -- Britain today ordered the expulsion of all but one of the remaining 16 Iranian diplomats here, and said it would reduce its own mission in Tehran to a single diplomat as relations between the two countries edged close to the breaking point.

A Foreign Office spokesman said that the 15 Iranians must leave the country by the end of this month. Shortly before the British announcement this morning, the official Iranian news agency IRNA said that the 15 were being "withdrawn" unilaterally by Tehran.

The British order is the latest in a series of tit-for-tat expulsions between the two governments that began last month after the shoplifting arrest of an Iranian consular official in Manchester. In apparent response, Iranian government agents abducted and beat up a British diplomat in Tehran.

A series of mutual expulsions followed, with Britain also recalling for precautionary reasons a number of its 19 diplomats stationed in Tehran. Only two diplomats remain at the British Interests Section in Tehran, and one of them, chief Christopher MacRae, is expected to return here, leaving only a junior officer as caretaker for the mission, operating under the Swedish flag.

The Foreign Office also advised an estimated 250 civilian British citizens living in Iran to consider "urgently" whether they could leave Tehran until the situation returned to normal. Travelers to Iran were advised to postpone their trips.

Officials here emphasized that today's action stopped short of a total break in diplomatic relations, a move Britain fears could worsen the prospects for up to 29 foreigners currently missing or held hostage in the region, including Church of England envoy Terry Waite.

Iran last year purchased more than $600 million in British exports and sold Britain about $150 million in goods.

Officials here said that Iran, while adopting a hard line, also seemed reluctant to break relations in apparent concern about operation of the Iranian National Oil Co. office here, which also serves as an arms procurement agency.