UK Identifies Legal Route for Military Action Against “Shadow Fleet” Vessels

The UK government has identified a legal mechanism that it believes would allow British Armed Forces to board and detain vessels operating as part of so‑called “shadow fleets,” according to information shared with the BBC. 

The move marks a significant escalation in efforts to counter ships linked to Russia, Iran, and Venezuela that are accused of sailing without valid national flags to evade international sanctions on oil.

The development comes days after British forces assisted the United States in seizing the Marinera, an oil tanker American officials say was carrying sanctioned crude for Venezuela, Russia, and Iran. 

Although no UK personnel boarded the vessel, RAF surveillance aircraft and the Royal Navy support ship RFA Tideforce took part in the operation. The tanker, previously known as Bella 1, was intercepted in the North Atlantic between Iceland and Scotland.

Officials say the government has spent several weeks examining what powers could be used to take direct action against unflagged or falsely flagged vessels. 

Ministers now believe the Sanctions and Money Laundering Act 2018 provides a legal basis for authorising the use of military force against such ships. The mechanism is being described internally as a key step in “ramping up” the UK’s response to the shadow fleet, though it remains unclear when the first UK boarding operation might take place.

The UK has already sanctioned more than 500 vessels it believes are helping to finance hostile activity, including Russia’s invasion of Ukraine. Ministers say coordinated action with allies has forced around 200 ships off the seas, many of which were operating without legitimate registration. 

Ship flagging - the process of registering a vessel under a recognised national flag - is essential for legal navigation in international waters and provides access to insurance and regulatory oversight. Shadow fleet vessels often operate without such protections, raising concerns about safety and environmental risks.

The government believes the newly identified legal powers could be applied to any sanctioned vessel lacking a valid flag, including the Marinera had the UK chosen to take the lead in that operation.

Defence Secretary John Healey told MPs that the UK is “stepping up action on the shadow fleet,” developing additional military options and strengthening coordination with allies. He said the joint operation with the US was “in full compliance with international law” and stressed that the UK “will not stand by as malign activity increases on the high seas”.

The US has intensified its own crackdown, seizing five tankers in recent weeks. 

Meanwhile, Transport Secretary Heidi Alexander said the UK has increased insurance checks on vessels sailing near British waters, with more than 600 ships stopped for inspection. Unflagged vessels typically lack insurance, prompting warnings from maritime experts that a major spill involving a shadow fleet tanker could trigger an environmental and financial crisis.

Alexander declined to say how many suspected shadow vessels had entered UK waters, arguing that disclosing the information “only helps one person and that is President Putin”.

Sanctions advice from Shearwater Law

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