A Royal Navy warship is offering increased protection to a second tanker in the Gulf in an apparent boost to maritime security following a threat from Iran to seize a British vessel, Sky News understands.

HMS Montrose, a Type 23 frigate, is travelling with the British Heritage tanker off the coast of the United Arab Emirates, according to a ship tracking website and a defence source.

The warship is not providing an official escort to the Isle of Man-flagged vessel, owned by BP shipping, but it is providing an increased level of monitoring.

British Heritage, which is able to hold more than one million barrels of oil, had earlier in the day been spotted sheltering off the coast of Saudi Arabia.

Image: British Royal Navy's HMS Montrose frigate moored at Harumi Pier earlier this year

HMS Montrose gave the same assistance to Pacific Voyager, also an Isle of Man-flagged tanker, for a "little way" over the past day before passing on the duty to another ship in the region, a second defence source said.


The source did not give the identity of the other ship but it would likely have been the warship of an allied nation also operating in the area.

Britain is expected to reveal details about what seems to be a change in posture towards commercial shipping in the Gulf in the coming days.

It is thought to be part of a push led by the United States for allies and other countries with tankers that use the Strait of Hormuz to bolster maritime security in the region following a spate of attacks against tankers blamed on Iran.

British ships have specifically been named as a target by Tehran after Royal Marines helped forces in Gibraltar seize an Iranian-flagged tanker allegedly bound for Syria in breach of European Union sanctions.

The capture of the Grace 1 tanker prompted Iranian President Hassan Rouhani today to warn the UK of "consequences".

"I point out to the British that you initiated insecurity (on the seas) and you shall grasp the consequences of it later on," he said in comments to the cabinet broadcast by state TV.

The president called the seizure of the tanker "a foolish act".

The 330 metre (1,000ft) Grace 1 tanker, capable of carrying two million barrels of oil, was halted on Thursday by police and customs in Gibraltar.

Iran condemned the detention as an "illegal interception," but Gibraltar officials said that the cargo was believed to be destined for Syria, which is subject to European sanctions.

Iran denied this, saying that the destination "was somewhere else".

Iran's defence minister vowed on Monday to respond to Britain's move.

Calling the tanker's seizure an act of maritime piracy, Brigadier General Amir Hatami said it "will not be tolerated by us and will not go without a response".

Gibraltar's supreme court ruled on Friday that the tanker can be held for 14 more days, the territory's attorney general said.

Tensions with Iran have heightened since President Donald Trump pulled the United States out of a nuclear deal with the regime and re-imposed sanctions.

A spokeswoman for the Ministry of Defence said HMS Montrose is in the Gulf region.

The last time the Royal Navy officially escorted ships in the Gulf was in the 1980s. The Armilla Patrol involved many more ships escorting groups of commercial vessels through the Gulf at a time of heightened threat from Iran.