Here is what we know so far.
At around 4pm UK time, the Stena Impero, was “was approached by unidentified small crafts and a helicopter during transit of the Strait, while the vessel was in international waters”, according to the tanker’s owners, Stena Bulk.
At the time of publication, the company said they had not been able to contact the crew, made up of Indian, Russian, Latvian and Filipino nationals.
Trackers showed the ship changing course and heading towards Iran.
There was no official statement from Iran about why the ship was seized but a port authority chief was quoted by the semi-official Tasmin news agency, as saying that it was “causing problems”. Unnamed sources were also quoted as saying that the ship was dumping residue.
The seizure appeared to take the British government by surprise. It said it was investigating.
Around an hour after the first ship was taken, a second tanker MV Mesdar was seized and was seen to veer some distance off-course towards the Iranian coast.
Describing the actions as “unacceptable”, Jeremy Hunt, the foreign secretary, convened a Cobra emergency committee of government ministers at Whitehall.
Speaking to reporters outside the White House, US President Donald Trump said America would be “working with the UK” and referenced reports that a second oil tanker may be involved.
“We will talk to the UK and we have no written agreement but we have an agreement. They’ve been a very great ally of ours.”
Bob Sanguinetti, of the UK Chamber of Shipping, also condemned the Revolutionary Guard’s move.
He said: “This incident represents an escalation. Whilst we call for measured response, it is also clear that further protection for merchant vessels must be forthcoming to ensure enhanced security to guarantee free flow of trade in the region.”
Around five hours after it changed direction, the MV Mesdar was allowed to continue its journey, it’s owner said, adding that it was boarded by armed guards. It did not state their nationality.
“All the crew are safe,” the company said in a statement.
After the Cobra meeting, a government spokeswoman said minsters were “deeply concerned about Iran’s unacceptable actions which represent a clear challenge to international freedom of navigation.”
She added: “We have advised UK shipping to stay out of the area for an interim period.”