Video report by ITV News Correspondent Emma Murphy
British-flagged ships will be escorted by the Royal Navy through the Strait of Hormuz, the Ministry of Defence has announced.
A Department for Transport spokesperson said: "In response to the heightened threat to commercial shipping in the Gulf, the Department for Transport changed its guidance on Friday 19th July to advise ships not to sail through the area.
"Freedom of navigation is crucial for the global trading system and world economy, and we will do all we can to defend it.
"The Department for Transport has therefore updated its advice to confirm that the Royal Navy has been tasked to accompany British-flagged ships through the Strait of Hormuz, either individually or in groups, should sufficient notice be given of their passage.”
In one of the first diplomatic challenges facing Boris Johnson, the new prime minister has been tasked with trying to calm tensions between the UK and Iran.
Tehran's foreign minister Mohammad Javad Zarif accused the British of “piracy” for seizing the Grace 1 supertanker which was seized on suspicion it was transporting oil to Syria in breach of European Union sanctions.
In a message congratulating the new Tory leader, Mr Zarif said: “Iran does not seek confrontation.
“But we have 1,500 miles of Persian Gulf coastline. These are our waters and we will protect them.”
The UK has also accused Iran of an “act of state piracy” over the seizure of the British-flagged Stena Impero by Iranian Revolutionary Guards last Friday.
Iran's state television agency released footage from on board the tanker which showed members of the 23-strong crew talking around a table and working in the tanker's control room and kitchen.