All the same, it is hard to read it as an entirely neutral act — a purely automatic response to an alleged breach of territorial rights. It is the kind of “offence” that could surely have been overlooked, if Iran chose to do so. It would be surprising if it had never happened before in the 16 months since the start of the Iraq war.

From Britain’s point of view, the move raises two worrying questions. Is it retaliation for Britain’s part in drafting the tough resolution on Iran’s nuclear programme, which was passed on Friday by the International Atomic Energy Agency (IAEA)? Is it also a warning that Britain cannot take Iran’s support for granted as its forces try to keep order in the south of Iraq prior to the handover of sovereignty? Iran offered yesterday only the bald details of the detentions: that three small British boats and their eight crew had been detained in what it claimed were Iranian waters.

Iran has made no secret of its anger at the rebuke last week in Vienna by the IAEA, the UN’s nuclear watchdog, in a declaration drafted by Britain, France and Germany, and backed by the US.

The IAEA board of governors’ report said that Iran’s co-operation had been patchy, and that it had not done enough to answer fears that its nuclear programme was designed ultimately to make weapons. Iran says it is interested just in civil power.

So it seems plausible that yesterday’s arrests were a warning shot that the Vienna declaration hurt relations with Britain, after eight months in which the nuclear issue has created tension. Jack Straw, the Foreign Secretary, has made a personal commitment to maintaining dialogue with Tehran, a marked difference from US policy.

There has been an historic pattern of tit-for-tat moves in Iran’s past relations with Britain. In 1987, when the British diplomat Edward Chaplin was beaten up by Revolutionary Guards in Tehran, it led to the expulsion of some Iranian diplomats from Britain, followed by the expulsion of British diplomats in Tehran.

Throughout this oscillation. Britain has tried to keep lines open. After Ayatollah Kho-meini’s “sentence of death” on the author Salman Rushdie in 1989, Britain responded with deliberate moderation.

There is a danger of overinterpreting yesterday’s move. If it is retaliation, it is curiously oblique. First, Iran has as much cause for complaint against France and Germany as it does against Britain, and even more against the IAEA and the US. Secondly, Iranian conservatives have made clear that they think the best retaliation is the most direct: by pursuing nuclear research with even more vigour.

Yesterday Iran’s supreme leader, Ayatollah Ali Khamenei, said that it was essential for Iran to produce its own nuclear fuel by enriching uranium so that it could not be put under pressure by foreign suppliers.

The process, while legal under the Nuclear Non-Proliferation Treaty, is the most controversial part of the programme as it could also give Iran the ability to make explosives.

“Iran is not trying to make an atomic bomb, because it does not need this to unravel its enemies”, Khamenei said, in a phrase often used to refer to the US and Israel. But he added that “if it bothers them (the Europeans) that Iran masters nuclear technology . . . we say to them that the Iranian people will never accept the language of force”.

The top national security official and nuclear negotiator Hassan Rowhani has already suggested Iran could resume uranium enrichment, a move which conservative MPs and newspapers now endorse.

Alaodin Borujerdi, the new conservative head of the parliament’s foreign affairs commitee, cautioned that MPs were unlikely to ratify the IAEA’s “additional protocol”, allowing it to make snap visits. Those hardline views on the nuclear programme, and the strength of conservatives in the new parliament, are the real problems Britain has to confront.

If there is comfort, there seemed little sign yesterday that Iran intends to change its strategy and complicate the work of British troops in Iraq by stirring up the Shia communities in the south. It appears content to keep playing a quiet game, waiting for the emergence of a fellow Shia government next door.

From a British point of view, Iran often sends mixed signals. To call yesterday’s move a warning might be to invest it with too much coherence or design. Yet it is hard not to read it as a sign that relations with Britain have cooled over the past eight months — and cooled even more in the past five days.

STRAINED RELATIONS

• February 1989: Iran breaks off diplomatic relations with Britain after Ayatollah Khomeini calls for the death of the author Salman Rushdie
• September 1998: The death sentence is lifted after talks between Robin Cook and Kamal Kharrazi, the Iranian Foreign Minister
• May 1999: Britain and Iran exchange ambassadors for the first time since 1989
• January 2000: Mr Kharrazi makes the first visit to London by a senior Iranian minister since 1979, meeting Tony Blair and Robin Cook
• September 2001: Mr Blair speaks on the telephone with President Khatami. Jack Straw makes the first visit by a British Foreign Minister to Iran since 1979, followed by three further visits in November 2001, October 2002 and June 2003
• September 2002: Iran accepts the appointment of Richard Dalton as British Ambassador to Tehran after rejecting the first candidate, claiming he was a Jewish spy
• 23 May, 2004: 400 student demonstrators attempt to storm the British Embassy in Tehran after several protests in the same week. No one is hurt

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