The British government raised its security warning for shipping in Iranian waters to its highest level as the Royal Navy was forced to fend off the attempted obstruction of a British oil tanker by Iranian Revolutionary Guards.

British flagged ships were notified at the beginning of this week that Iranian waters were considered a level three, or “critical” security environment, the Telegraph understands.

The security alert, which amounts to advice to avoid Iranian waters where possible and would have been accompanied by advice on specific precautions to take, came after Iran threatened “reciprocal” action for the recent seizure of an Iranian tanker by the Royal Navy near Gibraltar.

That action appeared to come on Wednesday, when the British Heritage, owned by BP Shipping and registered to the Isle of Man, was approached by three Iranian Islamic Revolutionary Guard Corps boats as it sailed through Persian Gulf towards the Strait of Hormuz.

The Iranians ordered the vessel to stop in nearby Iranian territorial waters, according to the Ministry of Defence, but withdrew after HMS Montrose, a Royal Navy frigate which had been escorting the tanker, aimed its guns on the Iranians and warned them to move away.