Iran’s taunting of the 15 British sailors and Marines held captive for more than a week is odious. The release yesterday of a third letter from the only woman among the group kidnapped in the Gulf makes obvious the intimidation torture being used to further Tehran’s propaganda war. Iran’s Revolutionary Guards may believe that parading a senior member of the Royal Navy crew on television to voice an “apology” for trespassing in Iranian waters will stir sympathy for the captives’ plight and force Britain into humiliating concessions to secure their release. The first assumption is correct: public opinion is outraged. But Iran is making a very big mistake if it thinks that it is winning the battle for world opinion. It could hardly be more isolated.

The UN Security Council vote expressing “grave concern” at Iran’s actions and calling for the group’s release stopped short of “deploring” the outrage, largely because Russia, which has its own reasons for treading carefully with Iran, watered down the motion. There are, nevertheless, clear signs that Moscow is exasperated with the maverick Tehran Government and may be seeking, like China, to distance itself from the extremists. In any case, the vote, coming soon after the unanimous demand that Iran cease its nuclear enrichment, shows that Iran now has no friends in the outside world. Even Iraq, whose Shia-led Government is anxious to maintain links with Tehran, has told its neighbour it is wrong.

That does not make it any easier for Tony Blair to translate this global frustration into effective leverage. His dilemma in his last months in office could even start to resemble the plight of Jimmy Carter, whose political humiliation as he tried vainly to negotiate the US hostages’ freedom, sealed his electoral fate. At least, however, Mr Blair has the robust support of his allies. European Union foreign ministers yesterday expressed their solidarity, while Javier Solana, who oversees EU foreign policy, warned Iran that it was making “a big mistake”. The Europeans’ reaction is important: France and Germany have been partners with Britain in the failed attempt to negotiate a nuclear deal. Both are angered by Tehran’s bad faith, and this latest provocation should persuade them to do the one thing that could hurt: restrict trade with Iran, which depends hugely on the EU, especially Germany, for vital imports. The EU agreed yesterday to consider “appropriate” further measures if needed.

Mr Blair is also looking to the Muslim world to underline Iran’s isolation. Any condemnation by Arab countries, themselves deeply worried by Tehran’s ambitions, would cut little ice. But isolation within the Organisation of the Islamic Conference would be a blow to a country that trumpets its championing of Muslim causes, even if the Revolutionary Guards, who are driving this crisis, may be hoping that outside pressure and isolation will bolster the hardliners at the expense of Iran’s frustrated pragmatists.

Turkey is well placed to voice Muslim concerns: indeed, Tehran’s shrill warning against invoking any “third party” shows the extremists’ fear that Turkey may take on the role that Algeria did to negotiate the eventual freedom for the US hostages. Western tactics must be to thwart the hopes and influence of these extremists. That means racheting up the pressure on Tehran, while pursuing an adroit diplomacy. There could, eventually, be a face-saving conference to delineate, once and for all, the waterway boundary, disputed since 1639. That is for the future. For now, Britain must make clear to Iranians as they enjoy a holiday that their country’s actions are despicable, unworthy of an old and proud civilisation.