The UK has raised the threat to British shipping in Iranian waters in the Gulf to the highest level.
The step was taken on Tuesday, amid growing tensions in the region.
Yesterday, Wednesday, Iranian boats tried to impede a British oil tanker in the region – before being driven off by a Royal Navy ship, the Ministry of Defence said.
Iran had threatened to retaliate for the seizure of one of its own tankers, but denied any attempted seizure.
The Department for Transport said it regularly provided security advice to UK ships in high-risk areas.