Britain's Prime Minister Keir Starmer leaves 10 Downing Street to attend the weekly session of Prime Minister's Questions in parliament in London, Wednesday, June 10, 2026. Kin Cheung/AP hide caption

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Kin Cheung/AP

LONDON — Britain is investigating a sanctioned tanker that is suspected of being part of the Russian "shadow fleet," shipping oil in violation of international sanctions over Moscow's war on Ukraine, British Prime Minister Keir Starmer said Sunday.

British armed forces boarded and detained the vessel, the Smyrtos, on Sunday in the English Channel, in what the country's Defense Ministry called "the first UK-led operation of its kind."

The vessel will be held and monitored off the south coast of England for investigation, according to a statement by the Defense Ministry. The operation was carried out "in close coordination" with French authorities, who have previously intercepted a number of vessels linked to the "shadow fleet."

Russia is believed to be using a fleet of hundreds of ships to evade sanctions over its war against Ukraine.

"This operation delivers yet another blow to Russia and reminds those fueling Putin's war in Ukraine that they cannot hide," Starmer said.

U.K. authorities said that such operations were "directly bearing down on the resources sustaining Russia's aggression in Ukraine and reducing its capacity to threaten security across Europe and beyond."