Introduction<\/p>\n
Since the full-scale invasion of Ukraine in 2022, Western governments have sought to constrain Russia\u2019s war economy through an expanding regime of financial and energy sanctions. In response, a so-called \u2018shadow fleet<\/a>\u2019 of ageing oil tankers, operating under opaque ownership structures and frequently changing flags, has emerged to circumvent these restrictions and sustain Russian exports. For policymakers in London and Washington, these vessels have become both a symbol of sanctions evasion and a practical challenge to enforcement at sea.<\/p>\n Against this backdrop, the UK Government has announced plans to deploy the Royal Navy and Special Forces to board and seize tankers suspected of breaching sanctions. Ministers insist that such operations are \u2018legally watertight\u2019. Yet this marks a significant shift: from the UK regulating access to its ports and financial system to physically interdicting vessels on the high seas.<\/p>\n This report examines whether that escalation is consistent with the legal order the UK claims to uphold. At its core lies a tension between domestic sanctions policy and the international law of the sea.<\/p>\n Can a state extend the reach of its unilateral sanctions beyond its territorial waters without eroding foundational principles such as freedom of navigation? And in attempting to close enforcement gaps, is the UK entering a grey zone where legal justification becomes increasingly contingent and contested?<\/p>\n Jurisdiction at sea: territorial waters and the EEZ<\/p>\n Under international maritime law, specifically the United Nations Convention on the Law of the Sea (UNCLOS) framework, the ocean is divided into distinct legal zones.<\/p>\n Within Territorial Waters \u2013 0\u201312 nautical miles (12 nautical miles (nm) is roughly 22.2 km from the coast) \u2013 the UK has clear sovereignty, according to UNCLOS (1982, Article 2<\/a>) and the Territorial Sea Act (1987<\/a>). It can enforce domestic laws, including the Russia (Sanctions) Regulations<\/a> (2019), exactly as it would on land. Simply put, if a ship is within the 12-nautical mile belt, it is legally inside the UK.<\/p>\n However, once a vessel crosses the 12-mile line, the legal landscape changes. The ship has now entered the Exclusive Economic Zone (12\u2013200nm). This is technically international waters and is where the UK only possesses rights over economic resources. This is detailed under UNCLOS (1982, Article 58<\/a>). In short, all states enjoying the freedom of navigation in the EEZ.<\/p>\n If a Russian-linked tanker remains outside the 12-mile perimeter but within the EEZ, on what legal basis can the Royal Navy justify an interdiction? Does a domestic sanctions list grant the UK the authority to override the international Freedom of Navigation?<\/p>\n To address this, the UK Government announced a policy shift on March 25 2026, authorising the Royal Navy and law enforcement to move toward interdiction of sanctioned vessels (Prime Minister\u2019s Office, 2026<\/a>). The Government\u2019s legal position can be backed by the idea that shadow fleet vessels, which operate with opaque ownership, can be reclassified as stateless vessels under UNCLOS (1982, Article 92<\/a>). This strips these ships of sovereign protection, allowing the Royal Navy to invoke the \u2018right of visit\u2019 under UNCLOS (1982, Article 110<\/a>) to board any vessel suspected of having no nationality.<\/p>\n As the UK Labour Government stated in Parliament: \u2018The legal basis is the fact that these ships are operating as either false flag or stateless vessels. That gives us the legal basis\u2026 for the actions that we either take or support.\u2019 (UK parliament, 2026<\/a>)<\/p>\n Despite this, questions arise. To some it appears that the UK is stretching its authority within the EEZ. Traditionally, it has been limited to resource management and until now not to enforcing sanctions. The UK could be accused of jurisdictional creep, which is when an authority slowly expands its power into areas where it isn\u2019t legally supposed to be. This is especially problematic where sanctions are recognised only by the UK and\/or the US rather than the UN.<\/p>\n It also sets a dangerous precedent: if the UK circumvents freedom of navigation today, it could lose its legal standing if other nations applied similar tactics against British vessels tomorrow.<\/p>\n Innocent passage and the limits of sanctions enforcement<\/p>\n Even within the 12-mile territorial limit, the Government\u2019s authority is not absolute. Under UNCLOS (1982, Article 17<\/a>), all foreign vessels enjoy the Right of Innocent Passage. The definition of \u2018innocence\u2019 is governed by UNCLOS (1982, Article 19<\/a>), which provides an exhaustive list of activities that render passage non-innocent. These include spying, weapons practice, or \u2018wilful and serious pollution.\u2019 However, and notably, being owned by a sanctioned entity is not on this list.<\/p>\n This raises a question: is being on a UK sanctions list sufficient to justify forced boarding?<\/p>\n Under the Russia (Sanctions) (EU Exit) Regulations (2019<\/a>), the UK has the power to specify ships and prohibit their entry into ports. However, as noted in the UNODC Maritime Crime Manual (2020, p. 27)<\/a>, the enforcement powers of a coastal state are limited by the \u2018Right of Innocent Passage.\u2019 The manual emphasises that while a state may prevent passage that is \u2018not innocent\u2019 under UNCLOS (1982, Article 25<\/a>), this is reserved for the aforementioned physical threats to security or safety.<\/p>\n Using domestic financial sanctions as a motive for forced boarding, without evidence of the prohibited acts listed in Article 19, could represent an expansion of state power that could violate the international legal order.<\/p>\n Professors Tom Mullen and Charlie Peevers<\/a> warn that the previous seizure of vessels such as Bella-1 have bypassed domestic laws (Mullen and Peevers, 2026). Under Section 137 of the Extradition Act (2003<\/a>), extradition requires \u2018dual criminality\u2019, meaning that the conduct must be a crime in the country that requests the seizure and in the country in which the seizure takes place.<\/p>\n In the case of Bella-1, the United States was the requesting country, and the seizure took place in the UK. However, there has been no Act of Parliament to authorise maritime seizures, either under Scots law or under any other part of the UK\u2019s legal framework, and furthermore, Bella-1 was not included in the UK Sanctions Act.<\/p>\n Testing the legal order: risks to rights and reciprocity<\/p>\n Beyond jurisdictional debates, the Government\u2019s shift carries implications for the international legal order and human rights.<\/p>\n Under UNCLOS (1982, Article 92<\/a>), a ship is subject to the exclusive jurisdiction of its Flag State while on the high seas. Changing flag during a voyage is strictly prohibited by this article, except in cases of a \u2018real transfer of ownership\u2019.<\/p>\n However, as Professor Douglas Guilfoyle<\/a> argued regarding the seizure of Bella-1, if a state like Russia granted a ship temporary nationality, it \u2018disposes of the issue\u2019 of statelessness. Guilfoyle warned that \u2018one state cannot challenge the validity of another state\u2019s grant of its flag to a vessel\u2026(and if so)\u2026 the result would be legal chaos\u2019 (Guilfoyle, cited in Akkad, 2026).<\/p>\n By ignoring these temporary flags to enforce sanctions, does the UK invite then a future where British-flagged vessels could be similarly targeted by hostile states in the name of national security?<\/p>\n In the M\/V SAIGA (1999, No. 2 Case)<\/a>, the International Tribunal for the Law of the Sea ruled that the use of force at sea must remain a last resort. The Tribunal\u2019s judgment (Paras 155-156) dictates that any enforcement action must be \u2018reasonable and necessary\u2019 and strictly guided by \u2018considerations of humanity.\u2019<\/p>\n In other words, boarding an uncooperative oil tanker in congested waters like the English Channel is a high-risk military operation. Any escalation could result in an environmental, not to mention diplomatic, disaster. This could expose the UK to liability.<\/p>\n The detention of a ship\u2019s crew, who are often third-country nationals with no personal link to the sanctioned entities, would also trigger obligations under the European Convention on Human Rights. These include Article 5 (Right to Liberty) and Article 6 (Right to a Fair Trial) and they apply even at sea if the vessel is under the control of UK forces.<\/p>\n If a crew is detained based on a contestable interpretation of maritime law, the UK could face lengthy legal battles. Is the UK prepared for the lega implications that could follow?<\/p>\n Conclusion<\/p>\n The UK Government\u2019s proposed shift from financial and territorial sanctions enforcement to physical interdiction at sea represents more than a tactical adjustment. It marks a potential reconfiguration of how unilateral sanctions interact with the international law of the sea.<\/p>\n As this briefing note has shown, the legal basis for such operations is, at best, contingent. Within the EEZ, the reliance on reclassifying vessels as stateless stretches the established limits of jurisdiction and risks conflating suspicion with legal status. Within territorial waters, the attempt to anchor enforcement in domestic sanctions law sits uneasily alongside the established doctrine of innocent passage, which does not recognise sanctions status as grounds for intervention.<\/p>\n Taken together, these tensions point to a broader concern: that the UK may be incrementally expanding its enforcement powers into areas where the legal foundation remains unsettled. This carries consequences beyond the immediate objective of disrupting Russian oil exports. It risks weakening the very norms, particularly freedom of navigation and flag state exclusivity, upon which the UK depends as a maritime trading nation.<\/p>\n There is also a practical dimension. Interdiction operations in congested waters raise the risk of escalation, environmental harm, and legal challenge. The detention of crews and seizure of vessels on contested grounds may expose the UK to sustained litigation, both domestically and before international tribunals. What is presented as a robust enforcement measure may, in practice, generate prolonged legal and diplomatic vulnerability.<\/p>\n Ultimately, the question is not whether sanctions should be enforced, but how far a state can go in doing so without undermining the legal order that gives those sanctions legitimacy.<\/p>\n Recommendations<\/p>\n First, the Government should seek to anchor any maritime interdiction policy in a clearer multilateral framework. Where possible, enforcement should be tied to UN-backed measures or coordinated regimes that reduce the perception of unilateral overreach and strengthen legal defensibility.<\/p>\n Second, Parliament should establish an explicit statutory basis for maritime seizures. The current reliance on executive interpretation and existing sanctions regulations leaves a gap in domestic legal authority that is likely to be tested in court.<\/p>\n Third, the threshold for classifying vessels as stateless should be clarified and subject to independent oversight. A presumption based on opaque ownership or flag-switching risks diluting the strict criteria set out under UNCLOS and invites challenge from flag states.<\/p>\n Fourth, enforcement policy should incorporate stricter operational safeguards. This includes limiting the use of force to clearly defined circumstances, prioritising de-escalation, and accounting for environmental risk in heavily trafficked waterways.<\/p>\n Finally, the Government should assess the reciprocal implications of its approach. Any expansion of enforcement powers at sea sets a precedent that other states may invoke. A strategy that secures short-term sanctions compliance at the expense of long-standing legal protections may, over time, prove strategically counterproductive and dangerous for civilian vessels.<\/p>\n SourcesAkkad, D. (2026) \u2018Did Trump and Starmer break international law with tanker seizure?\u2019, Declassified UK, 8 January. Available at: <\/a>https:\/\/www.declassifieduk.org\/did-trump-and-starmer-break-international-law-with-tanker-seizure\/<\/a> (Accessed: 28 March 2026).Extradition Act 2003, c. 41. Available at: <\/a>https:\/\/www.legislation.gov.uk\/ukpga\/2003\/41\/contents<\/a> (Accessed: 28 March 2026).International Tribunal for the Law of the Sea (1999) The M\/V \u201cSAIGA\u201d (No. 2) Case (Saint Vincent and the Grenadines v. Guinea), Judgment. ITLOS Reports 1999. Available at: <\/a>https:\/\/www.itlos.org\/en\/main\/cases\/list-of-cases\/case-no-2\/<\/a> (Accessed: 28 March 2026).Prime Minister\u2019s Office (2026) \u2018Policy update on maritime interdiction of sanctioned vessels\u2019, GOV.UK, 25 March.Russia (Sanctions) (EU Exit) Regulations 2019, SI 2019\/855. Available at: <\/a>https:\/\/www.legislation.gov.uk\/uksi\/2019\/855\/contents\/made<\/a> (Accessed: 28 March 2026).Territorial Sea Act 1987, c. 49. Available at: <\/a>https:\/\/www.legislation.gov.uk\/ukpga\/1987\/49\/contents<\/a> (Accessed: 28 March 2026).UK Parliament (2026) Hansard, HL Deb, 14 January, Vol 835. Available at: <\/a>https:\/\/hansard.parliament.uk\/<\/a> (Accessed: 28 March 2026).United Nations (1982) United Nations Convention on the Law of the Sea (UNCLOS). Montego Bay: United Nations. Available at: <\/a>https:\/\/www.un.org\/depts\/los\/convention_agreements\/texts\/unclos\/unclos_e.pdf<\/a> (Accessed: 28 March 2026).United Nations Office on Drugs and Crime (UNODC) (2020) Maritime Crime Manual: For Criminal Justice Practitioners. 3rd edn. Vienna: United Nations. Available at: <\/a>https:\/\/www.unodc.org\/documents\/maritimecrime\/Manual\/20-00100_Maritime_Crime_Manual_Second_Edition_ebook.pdf<\/a> (Accessed: 28 March 2026).<\/p>\n