The UK has raised the threat to British shipping in Iranian waters in the Gulf to the highest level - where the risk of attack is critical.
The threat level means British ships are advised not to enter Iranian waters.
The decision was made amid growing tensions in the region.
On Thursday, Iranian boats tried to impede a British oil tanker in the region, before being driven off by a Royal Navy ship.
Iran had threatened to retaliate for the seizure of one of its own tankers, but denied any attempted seizure.