The UK has authorised its military to board and detain Russian-linked oil tankers suspected of operating in the so-called shadow fleet.
Prime Minister Keir Starmer says the decision is designed to disrupt exports that help fund Vladimir Putin's war in Ukraine.
"We're going after his shadow fleet even harder," Starmer said, linking the decision to rising oil prices and ongoing global tensions.
British authorities said vessels that refuse to comply could be boarded, with potential criminal proceedings against owners and operators for sanctions breaches.
Russia has relied heavily on these ships to bypass Western restrictions since the 2022 invasion of Ukraine, with the UK estimating they transport most of its crude exports.
The announcement comes as European countries step up efforts to crack down on the network, amid concerns over both sanctions evasion and environmental risks.