LONDON — British forces intercepted a Russian shadow fleet oil tanker on Sunday morning, its Ministry of Defence said, in the latest effort to disrupt Moscow’s sanctions-evasion network.

Russia’s “shadow fleet” of more than 700 vessels is estimated to transport around 75% of the country’s sanctioned oil exports, providing a critical financial lifeline for the Kremlin and helping fund the missiles and drones used in its full-scale invasion of Ukraine.

“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 said in a statement.

Starmer shared a video on X showing troops descending onto the vessel from helicopters, conducting a search with guns drawn.

Royal Marine Commandos and officers from Britain’s National Crime Agency boarded the vessel, the Smyrtos, in the English Channel, the body of water separating Britain and France, the ministry said. It added that the tanker will be held and monitored off the south coast of England while investigations continue.

The operation lasted six hours and was conducted in close coordination with French authorities, the ministry said.

“Russia relies on its shadow fleet to fund its conflict in Ukraine, and our interdiction delivers a blow to Putin’s illegal war,” Britain’s Defence Secretary Dan Jarvis said.

Ukraine’s President Volodymyr Zelenskyy said he was “grateful to the UK for taking this important step against Russia’s oil fleet,” adding in a post on X: “It was Russia’s hubris, fueled by high oil and gas revenues, that paved the way for this war, and every decision by partners that deprives Russia of money also limits the war itself.”

According to vessel-tracking website MarineTraffic, the Smyrtos sails under the flag of Cameroon. The vessel is currently anchored off the English seaside town of Weymouth.

The English Channel is one of the world’s busiest shipping lanes.

Britain and France have both pledged to obstruct vessels linked to Russia’s sanctioned “shadow fleet” that pass through their waters carrying sanctioned Russian oil or other goods for sale on black markets elsewhere, helping Russian President Vladimir Putin finance his war effort in Ukraine.

The U.K. has sanctioned almost 600 shadow fleet vessels to date, the ministry said, adding that Russia’s oil revenues are down 27% from October 2024 levels and are now at their lowest point since the start of the war.

But enforcement efforts against Russian vessels that rely on the waterway have only recently been stepped up.

At the end of May, the French navy in the Atlantic boarded an oil tanker named the Tagor that was subject to international sanctions and sailing from Russia, according to French President Emmanuel Macron.

In January, France, acting on intelligence provided by the U.K., intercepted another tanker in the Mediterranean that had travelled from Russia. French maritime authorities said at the time that the vessel, the Grinch, was suspected of operating under a false flag.

U.K. Defense Secretary John Healey unexpectedly quit on Thursday, saying the government is unwilling to spend enough on the military at a time of “rising threats,” citing the Iran war, Russia’s all-out invasion of Ukraine and threats from Moscow.

Starmer has pledged to boost U.K. defense spending to 2.5% of gross domestic product by 2027 and 3% by 2035, but many in the military say spending isn’t rising fast enough to meet growing threats.

“You have been unable, and the Treasury has been unwilling, to commit the resources that the nation needs to defend the country at this time of rising threats,” Healey wrote in his resignation letter.