Britain has raised the security alert to British ships in Iranian waters in the Gulf to its highest level.
The security code was increased to level three, which means the risk of attack is “critical, in the early hours of Tuesday.
British-flagged ships were advised to stay alert for Iranian patrol boats “being aggressive,” a source told Sky News.
It comes after police in Gibraltar arrested the captain and chief officer of the detained Iranian Grace 1 supertanker accused of violating EU sanctions on Syria.
And on Wednesday, a Royal Navy frigate trained its guns on three Iranian boats seeking to force an oil tanker off its route.
The military clash took place late evening as the tanker was travelling through the Strait of Hormuz.
Three Iranian vessels were believed to be manned by forces from Iran’s Islamic Revolution Guard Corps.
They were spotted speeding towards the BP-operated British Heritage tanker and reportedly ordered it to change course.
But HMS Montrose, a Type 23 frigate on maritime security duty in The Gulf, stepped it to protect the ship.
The Iranian boats backed off after being warned by radio not to harass the tanker.
The incident came 24 hours after Iran’s president warned Britain it would face “consequences” over the seizure of an Iranian oil tanker.
Iran has demanded the immediate release of the Grace 1, which British Royal Marines boarded off Gibraltar last week.