Britain’s armed forces have intercepted and seized control of a Russian shadow fleet oil tanker sailing in the English Channel, the British defense ministry said on Sunday.
Royal Marine Commandos and specially trained law enforcement officers boarded the vessel early Sunday in a military operation that lasted six hours and that was supported by British military ships and aircraft, the defense ministry said in a statement.
The intercepted tanker, the Smyrtos, will be held and monitored off the southern coast of England, it added.
“This operation delivers yet another blow to Russia and reminds those fueling Putin’s war in Ukraine that they cannot hide,” Prime Minister Keir Starmer of Britain said in a statement, referring to President Vladimir V. Putin of Russia.
Russia’s shadow fleet is a collection of often dilapidated ships with hazy ownership that covertly moves fuel around the globe, enabling the country to sidestep international sanctions imposed after its invasion of Ukraine.
According to the British government, the shadow fleet consists of more than 700 vessels and is responsible for carrying 75 percent of Russia’s sanctioned oil, giving the Kremlin an important economic lifeline.
Britain, which has so far imposed sanctions on more than 500 Russian shadow fleet vessels, says more than 70 percent of them are over 15 years old.
The interception of the Smyrtos was the first time that British forces had acted alone to stop a ship in the shadow fleet, the defense ministry said.
Earlier this year, the British military assisted the United States in seizing an oil tanker, the Marinera, in the waters between Iceland and Scotland. American officials said the ship had violated sanctions by carrying oil for Venezuela, Russia and Iran.
After that operation, the British government had said it was exploring how British forces could take similar action against sanctioned vessels traveling through its waters. In March, Mr. Starmer decided British armed forces and law-enforcement officers could board shadow fleet vessels in accordance with international law, the defense ministry said.
The operation against the Smyrtos was conducted in “close coordination” with France, building on recent collaboration between the two nations.
Earlier this month, President Emmanuel Macron of France said his country had intercepted an oil tanker thought to be part of the Russian shadow fleet. That vessel, the Tagor, was detained with British support in the Atlantic Ocean, around 400 nautical miles west of Brittany. It was the fourth suspected shadow fleet vessel that France has boarded since September 2025, according to the French authorities.
Britain’s military action comes at a sensitive moment politically. The country’s defense secretary, John Healey, and the armed forces minister, Al Carns, both quit their posts last week in a dispute over military funding plans.
A new British defense investment plan is expected to be published before a NATO summit scheduled for next month. But when he left the government, Mr. Healey warned that the level of military spending proposed by Mr. Starmer “falls well short” of what is needed to protect Britain.
That political rift came against a backdrop of growing tensions between Britain and Russia, including a series of incursions by Russian ships and aircraft around the British coast. British officials believe these were designed to test Britain’s military capabilities or to map critical underwater infrastructure, including cables.
Mr. Carns, the former armed forces minister, told the BBC on Sunday that British forces had not previously boarded a Russian shadow tanker partly because “we had a Russian frigate in the Channel protecting some of those ships coming through.”
“It was about hitting the right parameters to make sure that everything — from legal to the cargo — met the requirements for boarding,” he said, while adding that more boardings were likely in the future.
