For more than four days British sailors and Marines have been imprisoned in Iran. They have been interrogated, psychologically abused, denied access to the outside world and pressured into giving “confessions”. The 15 were seized at gunpoint by armed Iranian Revolutionary Guards while carrying out the thankless task of routinely searching shipping in Iraqi waters in the Shatt al-Arab waterway, the southern boundary between Iraq and Iran. Their kidnapping is an outrage. In earlier times it would have been an immediate casus belli. It would fully justify the use of force to obtain their release. There is, however, an even greater outrage compounding this insult to international law: the pusillanimous timidity of British officials and politicians, who have failed disgracefully to confront Iran with the ultimatum this flagrant aggression demands.

The Foreign and Commonwealth Office said it was “monitoring the situation very carefully and taking the situation very seriously”. Britain’s Ambassador in Tehran had a “businesslike” meeting with Iranian officials. Tony Blair has muttered that the issue was “fundamental” for his Government. The Iranians must be quaking at such threats. With the hubris and hypocrisy of a regime attempting to conceal its guilt, Iranian officials insist that the captured men are being well treated but their case must now follow “due legal process”. What “well treated” means can well be imagined: some of the Britons who were seized in a similar incident three years ago have described the mock executions, the psychological torture and the intimidating way that their captors tried to force admissions of guilt. As for “due legal process”, the denial of consular access, the refusal to provide evidence of trespass and the removal of the men to an unknown location hardly suggest the norms of international law.

Iran has already let slip its motives and intentions. The shrill denial that the men were being held hostage or that their case had any connection with the five Iranians detained by the Americans in northern Iraq in January only confirms suspicion that the Revolutionary Guards, who have provided funds, weapons and training to Shia militias, see the Britons as pawns to bargain for the Iranians’ release. They may also have been responding to the defection six weeks ago of a former head of the Revolutionary Guards.

The Britons’ abduction was clearly premeditated and the ambush carefully set. The perpetrators perhaps also thought — such is the naivety of narrow-minded fanatics — that this would influence the UN Security Council and prevent the passage of a second wave of sanctions over Iran’s refusal to halt its uranium enrichment programme. Both scenarios demonstrate the contempt in which Iran’s extremists hold world opinion.

It is all the more depressing, therefore, that the Western response has been so feeble. Diplomats hint at a “face-saving” solution. Analysts point to splits and divisions in Tehran. Some misguided “understanding” of the kidnapping seems to inhibit any response that may exacerbate tensions. This is precisely the wrong message. It encourages Tehran’s hardliners and probably prolongs the bargaining over the men’s detention. Even the Shia-dominated Iraqi Government has called on Iran to release the men — a far bolder call than anything coming from London or Washington. The coalition cannot allow Tehran to intimidate its neighbour. It must set a deadline for the men’s release and tell Iran bluntly that its piracy justifies immediate and more drastic sanctions.