The seizure of a British-flagged tanker by the Iranian Revolutionary Guard is a deliberately provocative act that is in breach of international laws.
It marks an unwelcome and worrying escalation in the simmering dispute between London and Tehran.
The UK government must now respond forcefully while treading carefully. The first priority is to ensure the safety of the crew.
We know from the despicable way Nazanin Zaghari-Ratcliffe has been treated that Iran has little care for human rights.
Then we must consider how we can protect our shipping in one of the most vital trade routes in the world.
If oil tankers cannot sail through the Straits of Hormuz there is a risk to fuel supplies in this country with potentially difficult consequences for industry and motorists.
Any response must be carefully calibrated. If we are too heavy handed then it could inflame tensions with the volatile Iranian regime.
Equally, if we fail to stand up to their aggression then it could only further embolden them.
Iran had the chance to ditch its status as a rogue state and rejoin the international community. The irony is that Britain was their ally on this journey.
The inflammatory behaviour of Tehran in recent months suggests this is a lost cause.
