British Prime Minister Keir Starmer said on Wednesday he has given the military permission to board and detain Russian ships his government alleges are part of a network of vessels that enables Moscow to export oil despite Western sanctions.

The decision comes as other European nations have stepped up efforts to disrupt Russia’s so-called shadow fleet of tankers used by Moscow to fund its four-year war against Ukraine.

Starmer said he approved more aggressive action against the vessels because Russian President Vladimir Putin was likely “rubbing his hands” at the sharp rise in oil prices driven by the US-Israel war against Iran.

“That’s why we’re going after his shadow fleet even harder, not just keeping Britain safe but starving Putin’s war machine of the dirty profits that fund his barbaric campaign in Ukraine,” Starmer said in a statement.

Downing Street said British military and law enforcement officials have been preparing to board Russian vessels that do not surrender, are armed, or use high-tech pervasive surveillance to evade capture.

Once the ships are boarded, Downing Street said criminal proceedings may be brought against the owners, operators and crew, for breaches of sanctions legislation.

Russia’s reliance on the shadow fleet has allowed it to keep exporting oil without complying with Western restrictions imposed after its full-scale invasion of Ukraine in 2022.

European efforts to keep up pressure on Russia were undermined this month when US President Donald Trump’s administration gave countries a 30-day waiver to buy sanctioned Russian products currently stranded at sea to stabilise global energy markets roiled by the war in Iran.

Starmer made the announcement before he attends the Joint Expeditionary Force Summit in Helsinki on Thursday, where Britain will call for more coordination on seizing shadow fleet vessels.

Britain has placed sanctions on 544 Russian shadow fleet vessels. They occasionally travel through the Channel, the water that separates Britain and France.

About three-quarters of Russia’s crude oil is transported by these ships, Britain estimates. The shadow fleet vessels typically have opaque ownership structures and have raised concern about environmental risks, with poorly regulated, ageing tankers prone to spills, mechanical failures and leaks, threatening marine ecosystems.