Via CNN: https://edition.cnn.com/2019/07/10/politics/iran-attempted-seize-british-tanker/
This is probably a response to last Thursday, when British Royal Marines seized an Iranian oil tanker near Gibraltar https://www.chronicle.gi/syria-bound-oil-tanker-carrying-iranian-crude-is-seized-off-gibraltar/Five armed Iranian Islamic Revolutionary Guard Corps boats unsuccessfully tried to seize a British oil tanker in the Persian Gulf Wednesday, according to two US officials with direct knowledge of the incident.
The British Heritage tanker was sailing out of the Persian Gulf and was crossing into the Strait of Hormuz area when it was approached by the Iranian boats. The Iranians ordered the tanker to change course and stop in nearby Iranian territorial waters, according to the officials. A US aircraft was overhead and recorded video of the incident.
The UK's Royal Navy frigate HMS Montrose had been escorting the tanker from the rear. It trained its deck guns on the Iranians and gave them a verbal warning to back away, which they did. Montrose is equipped on the deck with 30 mm guns specifically designed to drive off small boats.
Gibraltar port and law enforcement officers, assisted by British special forces, last night boarded and seized a supertanker reportedly carrying Iranian crude oil bound for Syria in breach of EU sanctions.
The Panama-flag very large crude carrier Grace 1, which was carrying two million barrels of crude oil, was boarded 2.5 miles off Gibraltar just before 2am on Thursday morning.
The vessel and its cargo have been detained, the Gibraltar Government said in a statement.
The Gibraltar authorities said they had information giving “reasonable grounds” to believe the vessel was acting in breach of European Union sanctions against Syria.