Current contractual clauses were written on the assumption of an ongoing united front, a core premise that has just collapsed.

In practice, British and European insurers may not be able to follow a US lead unless the US party adopts the UK and EU stance.

The complications apply to shipowners too. Those in the UK and the EU are forbidden by statute from following US secondary sanctions. But if Trump is feeling capricious — and when doesn’t he? — he could choose to apply the policies anyway. Damned if you do, damned if you don’t.

Meanwhile, it seems to be that the clock is about to turn back on sanctions with Iran. The UK, France and Germany — acting collectively under the moniker of E3 — have indicated displeasure with the latter’s continued drive for the bomb.

Israel and the US feel free to conduct extra-judicial assassinations of Iranian atomic scientists and to blow up uranium enrichment plants. London, Paris and Berlin obviously won’t go that far.

Instead, they are set to invoke the “snapback” clauses in the Joint Comprehensive Plan of Action sanctions relief deal signed by Tehran and six leading powers in 2016, probably in October.

This is more by way of a symbolic show of support for Jerusalem and diplomatic alignment with Washington than anything that would make much difference in the real world.

While it would make it even harder than it is now for shipowners to trade with Iran and for marine insurers to cover them, volumes are already close to zilch.

But beware of the law of unintended consequences. Snapback may simply push Iran closer to Russia, thereby negating much of the object of the exercise.

Iran still boasts some sizeable domestic fleets, including Islamic Republic of Iran Shipping Lines and National Iranian Tanker Co.

For decades, even after the Islamic revolution that brought the ayatollahs to power in 1979, these were happily entered with British P&I clubs. But no more, and not again, at least any time soon.

Once again, there is a prospect of regulatory divergence. The EU was not a party to JCPOA, leaving shipowners outside the E3 facing uncertainty about the hoops they must jump through.

Quite simply, we should not be where we are now. Politicians often boast about their ability to devise what Westminster jargon brands ‘joined-up policies’. That handwriting should extend to the multilaterally as well as domestically.

Prosecutions under the blocking legislation have been thankfully rare, at least in the UK. But we are not characters in a Joseph Heller novel; the very existence of this gratuitous Catch-22 creates uncertainties shipowners and marine insurers could live without. The conundrum should be resolved, one way or the other.

If Western governments are going to tie shipowners’ hands behind their backs, the very least they can do is to lay down clear, consistent and congruous rules.