The United Kingdom is preparing to expand its capacity to act against so-called “shadow fleet” oil tankers suspected of circumventing sanctions on Russian, Iranian, and Venezuelan oil exports. British ministers have identified a domestic legal framework that could allow the military to board, detain, or divert certain vessels operating without valid national flags, after financial and monitoring measures have proven insufficient to curb maritime sanctions evasion.
Officials stressed that no UK military boarding operations have yet taken place, but preparatory work has been underway for several weeks to determine how Royal Navy forces could be used to enforce shipping sanctions against unflagged or falsely flagged vessels. The move reflects growing concern that existing sanctions and monitoring mechanisms have not halted continued illicit maritime activity, particularly the movement of oil linked to sanctioned states.
The legal basis for potential action is rooted in the Sanctions and Anti-Money Laundering Act 2018. Provisions within the legislation allow authorities to detain, control, or restrict the movement of ships identified as disqualified or operating under false or illegitimate registration. The Act also enables enforcement measures against vessels owned, operated, or controlled by designated individuals or entities connected with sanctioned states. UK ministers believe these clauses can be interpreted to authorize military involvement, including the use of force if necessary, against unflagged or falsely flagged tankers. According to officials, such vessels fall outside the legal protections typically afforded at sea and can therefore be treated as disqualified ships under UK law.
A UK government spokesperson described the interpretation as an escalation in enforcement tools rather than a shift in sanctions policy. Future operations could see Royal Navy surface vessels stopping and detaining ships, supported by surveillance aircraft and, if authorized, special forces boarding teams. Potential actions could include controlled boarding, diversion to port, or enforced removal from UK waters. However, ministers emphasized that political authorization and detailed operational planning would be required before any military measures are taken.
The UK’s reassessment comes amid a more assertive approach by the United States, which has moved from enforcement of financial and regulatory sanctions toward direct interdiction at sea. Over recent weeks, US authorities have seized five oil tankers linked to sanctioned oil flows from Russia, Iran, and Venezuela. Notable examples include the Marinera (formerly Bella 1), intercepted in the North Atlantic with UK surveillance and maritime support, as well as the Olina and the Skipper, vessels accused of attempting to move sanctioned Venezuelan or Russian oil under false or rapidly changing flags. In many cases, these ships were evading sanctions by reflagging to states such as Guyana, Timor-Leste, or Russia, or by operating without valid insurance and opaque ownership arrangements.
The US seizures represent a significant shift in sanctions enforcement, moving beyond port restrictions and financial penalties to physically removing vessels from service when other measures have failed. UK officials have confirmed that countering the Russian shadow fleet is a government priority, and that lessons from US operations are informing UK planning.
Shadow fleets, as described by Western authorities, are networks of oil tankers designed to sustain exports from sanctioned states. These fleets have expanded significantly since 2022, often comprising older vessels purchased on secondary markets after Western owners exited Russian, Iranian, or Venezuelan trades. Shadow fleet tankers commonly change names and flags, sometimes multiple times a year, and often register under jurisdictions with limited oversight, including Guyana, Panama, Liberia, Timor-Leste, or Russia itself. This practice allows sanctioned states to maintain oil exports despite price caps, insurance restrictions, and port denials.
Several high-profile vessels illustrate this pattern. The Marinera, formerly Bella 1, and the Olina repeatedly altered their names and flags while continuing sanctioned trade routes. Estimates from allied governments suggest that the global shadow fleet comprises more than 1,000 vessels, a number that has steadily grown as sanctions tightened. Many of these ships operate without coverage from major insurers, relying instead on non-recognized or opaque insurance arrangements—or operating without any coverage at all. This exposes coastal states to significant financial and environmental risks in the event of accidents, such as collisions, groundings, or oil spills. Past incidents in the Baltic Sea, North Sea, and Mediterranean have reinforced concerns about safety standards and maritime hazards posed by these fleets.
The UK government reports that it has imposed sanctions on over 500 vessels linked to shadow fleet networks. Coordinated action with international allies has reportedly forced approximately 200 of these ships out of active service, either through seizure, denial of port access, loss of insurance, or other logistical disruptions. Alongside sanctions, UK authorities have intensified checks in surrounding waters, stopping more than 600 ships near the British Isles to verify compliance and insurance coverage. Officials noted that a significant proportion of inspected vessels lacked valid insurance or clear flag registration, highlighting risks to UK coastal waters and shipping lanes.
Beyond maritime enforcement, the UK has expanded sanctions targeting the Russian oil sector itself. British authorities have joined the US in sanctioning major Russian energy companies, including Gazprom Neft and Surgutneftegas, alongside traders, insurers, and shipping entities involved in oil exports. Hundreds of tankers have also been designated under these measures, restricting their access to finance, insurance, and ports. The combined goal is to disrupt revenue streams from Russian oil sales and weaken the logistical networks sustaining sanctioned exports.
Officials emphasize that financial sanctions alone have proven insufficient to halt shadow fleet operations, prompting the UK to pursue a multi-layered approach combining financial measures, maritime enforcement, and potential military action. While military seizure remains a contingency, the strategic intent is clear: to close gaps in enforcement that have allowed sanctioned oil to continue moving globally despite international restrictions.
Observers note that the UK’s potential move toward military involvement in sanction enforcement reflects broader shifts in international approaches to sanction evasion. With shadow fleets increasing in size, sophistication, and reach, authorities in the UK, US, and EU face mounting challenges in monitoring and intercepting vessels operating outside conventional regulatory systems. The UK’s readiness to explore military options underscores a willingness to integrate naval power, surveillance capabilities, and legal authority to confront a persistent maritime challenge.
Experts caution, however, that military operations against unflagged or falsely flagged vessels are legally and operationally complex. Issues of jurisdiction, rules of engagement, and potential diplomatic fallout must be carefully managed. Moreover, the dispersed nature of shadow fleets, often operating in distant international waters, complicates both detection and interdiction. Despite these challenges, UK officials argue that the risk of inaction—allowing sanctioned oil to continue flowing—outweighs potential operational difficulties, particularly when coordinated with allied enforcement efforts.
In the coming months, UK authorities are expected to continue refining legal interpretations, operational planning, and coordination with allies to enable rapid action if required. While ministers have not specified timelines for potential military operations, the preparatory work signals a significant escalation in the UK’s ability to counter sanctions evasion at sea. Officials emphasize that any action would follow careful political and operational authorization and aim to enforce compliance with international sanctions while minimizing risks to personnel and commercial shipping.
The combination of financial, regulatory, and potential military measures illustrates the evolving approach to maritime sanctions enforcement. As shadow fleets grow in both size and sophistication, the UK, alongside international partners, appears prepared to adopt a more assertive posture to ensure that sanctions regimes have meaningful impact, safeguarding both the integrity of international sanctions and the security of critical maritime routes.
