British military units are poised to board and detain ships from Russia's so‑called shadow fleet after the prime minister authorised action against vessels operating near UK shores.
The shadow fleet is accused of sailing without proper flags to sidestep sanctions and move oil that helps finance Moscow's war in Ukraine.
Ministers say they identified a legal route in January under the Sanctions and Money Laundering Act 2018, clearing the way for military interventions in UK waters.
Officials expect the first boarding operation to take place soon.
Specialist forces have spent weeks rehearsing scenarios, including the risk of armed resistance from crews.
Depending on the threat level, the Special Boat Service and Royal Marines could lead the interventions.
The Royal Navy has been assisting partners such as Finland, Sweden and Estonia with monitoring and tracking these vessels in recent weeks.
Ship‑tracking systems can flag sanctioned ships days or even weeks before they reach British waters, allowing planners to pick targets in advance.
Government figures say roughly three quarters of Russia's crude is carried on older tankers and that 544 vessels have been sanctioned as part of efforts to cut off illicit revenue.
Earlier this year the UK supported US forces in seizing the oil tanker Marinera, an operation that prompted ministers to pursue similar UK‑led actions.
Defence officials and lawyers have since been ironing out the legal and operational details.
Ahead of a Joint Expeditionary Force summit in Helsinki, the prime minister framed the effort as part of a wider push to deprive President Putin of the oil profits that sustain military aggression.
Defence and legal chiefs also discussed the plan with JEF partners to ensure clarity on different scenarios.
The move has drawn political debate at home.
Conservatives welcomed attempts to block sanctioned ships but criticised Labour over military resources.
Independent analysis has also pointed out that numerous sanctioned vessels passed through the English Channel after the legal option was identified, highlighting the practical challenges ahead.
Ministers say vigilance will continue even as other global crises command attention, and military planners are working on the assumption that boarding operations will begin sooner rather than later.
