An MOD spokesman said HMS Montrose was "forced to position herself" between the three boats and commercial ship British Heritage before issuing verbal warnings to the Iranian vessels.
He said the Iranian boats "attempted to impede" British Heritage in actions that were "contrary to international law".
The spokesman said the MOD was "concerned" by the incident and called on Iran to "de-escalate the situation in the region".
Serious escalation: Iranian vessels attempted to seize a UK oil tanker overnight.— Lucy Fisher (@LOS_Fisher) July 11, 2019
MoD confirms three IRGC boats approached British Heritage, BP-owned tanker, in Gulf.
Royal Navy warship HMS Montrose, came between & issued verbal warning, prompting Iranian boats to back off. pic.twitter.com/I5BQXQPVvi
American officials, quoted by the US media, said HMS Montrose "pointed its guns at the boats and warned them over the radio".
HMS Montrose had been shadowing another British tanker, the Pacific Voyager, as it travelled through the Strait of Hormuz.
Royal Marines boarded an oil tanker in Gibraltar on its way to Syria.
The incident comes a day after Iran warned that Britain will face "repercussions" following the seizure of an Iranian supertanker by Royal Marines near Gibraltar. (end of excerpt)
Click here for the full story, on Forces Network website.