Iran demonstrated in 2007 its willingness to provoke Britain when its forces arrested 15 British naval personnel in the Shatt al-Arab ("Arvand Rood" in Persian) waterway . They were accused of straying into Iranian waters and held for 12 days.

The reasons for the regime’s latest behaviour are well understood by British diplomats, but senior officials admit that it is difficult to assess how far and how fast Iran is prepared to go to distract domestic attention from a crisis that has taken it by surprise.

With no US diplomatic representation in Iran, it is to some extent inevitable that Britain is accused of stirring up discontent. The predisposition even of educated Iranians to believe stories about “foreign meddling” should not be underestimated, British officials say.

Mr Miliband is determined not to give the regime’s diehards “gratuitous ammunition”. But he also knows that he cannot allow to give the impression that he is being cowed and that the intimidation is working.

Diplomats say that the arrests fit into a well established pattern of behaviour — starting with the recent condemnation by Ayatollah Khamenei, the Supreme Leader, of Britain as the “most evil” foreign power.

Staff working for the embassy had reported being followed or pushed in the street in the days leading up to the detentions. The expulsion of two British diplomats last week confirmed the regime’s intent, officials say.

One obvious, if dramatic, way to increase the tension would be the expulsion of the British Ambassador to Iran. Although there has been some speculation in the Iranian media to this effect, officials think it unlikely.

Mr Miliband clearly believes it wise to get his European allies behind him before any future confrontations. The statement issued by EU foreign ministers after a meeting in Corfu yesterday threatened the Iranians “with a strong and collective EU response” to further intimidation of diplomats.

The Foreign Secretary said: “All European countries have made clear that they want to stand together in standing up for the diplomatic principles that are important for our diplomatic activity all over the world.”

In private, however, British officials are less sanguine about the extent of EU solidarity, with Germany, Ireland and Greece considered the least likely to maintain a united front. Italy — which had in the past sided with those nations inclined to be tolerant of the Iranians — has now adopted a tougher posture, British diplomats say. Such a shift could be significant if Iran dominates the meeting of the G8, hosted by Silvio Berlusconi, early next month.