Seizing the shadow fleet

The 'shadow fleet'

Following Russia’s full-scale invasion of Ukraine in 2022, the UK and its G7 allies imposed unprecedented sanctions on the Russian energy sector, including an Oil Price Cap (OPC). To evade these restrictions and maintain its primary revenue stream, Russia has increasingly relied upon a ‘shadow fleet’ to export its oil.

This fleet consists of hundreds of ageing, obscurely owned, and often uninsured vessels (PDF) operating outside standard maritime regulations (for example, by flying under a false flag). In addition to undermining the sanctions regime, these ships are considered to pose significant environmental and maritime safety risks (PDF).

This briefing outlines the domestic and international legal frameworks that would govern how the UK could enforce sanctions against the shadow fleet within UK territorial waters and beyond.

Enforcement actions against the shadow fleet

In response to the shadow fleet, the UK and its allies have escalated their enforcement measures. In March 2026, following support for a US-led seizure of a stateless vessel, the Marinera, in January, the UK Government announced plans to actively intercept shadow fleet vessels transiting UK waters. Official statements have described this as ‘interdiction’.

This policy culminated in the first direct interception by UK forces on 14 June 2026, when Royal Marine Commandos and National Crime Agency officers boarded and detained the sanctioned oil tanker Smyrtos in the English Channel.

The legal framework around intercepting ‘stateless vessels’

The interdiction of shadow fleet vessels relies upon the UK’s domestic sanctions framework and the United Nations Convention on the Law of the Sea (UNCLOS).

To date, enforcement actions, including the recent interceptions of the Smyrtos and the Marinera, have occurred in international waters. “International waters” is not a legal term but is used to describe areas beyond a state’s territorial sea where freedoms of navigation usually apply, including a state’s exclusive economic zone. In these areas, UNCLOS Article 110 ‘right of visit’ permits warships to board vessels (PDF) that are reasonably suspected of being ‘stateless’ or sailing under false flags. Article 58(2) of UNCLOS also extends rules of the high seas to a state’s exclusive economic zone (EEZ) – this includes freedom of navigation, but also the right of visit under Article 110.

Once a vessel is determined to be stateless in international waters, the UK argues that it is lawful to use domestic maritime enforcement powers:

UNCLOS Article 110 permits a warship to exercise a right of visit to verify the flag where there are reasonable grounds to suspect the vessel is without nationality. Where a vessel is determined stateless, the UK can exercise powers available under its domestic legislation. Potential domestic powers include the ship-sanctions provisions in the Russia (Sanctions) (EU Exit) Regulations 2019, criminal and maritime enforcement powers under the Policing and Crime Act 2017.

Domestic sanctions and maritime enforcement powers

The domestic legal framework is governed primarily by the Sanctions and Money Laundering Act 2018 and the Policing and Crime Act 2017. These Acts provide the government and maritime enforcement officers with extensive powers, including the ability to issue directions to detain vessels, prevent port entry, and physically stop, board, and search ships suspected of sanctions violations.

However, the exercise of these domestic powers against foreign vessels, particularly within the UK’s territorial sea, is heavily constrained by international law and either requires the consent of the vessel's flag state or a legal basis under UNCLOS. Enforcement action is therefore more availble against 'stateless' vessels.

Emerging legal challenges and ‘innocent passage’

Because Article 110 applies exclusively to the high seas, it remains to be seen what legal basis the UK would rely upon to seize a shadow fleet vessel within its domestic waters.

In domestic waters, UNCLOS guarantees foreign vessels the right of ‘innocent passage’, provided their transit does not prejudice the peace, good order, or security of the coastal state. There are therefore significant legal ambiguities around the lawfulness of enforcement within the territorial sea.

Legal commentators remain divided on whether the transportation of sanctioned oil or the broader strategic threat posed by the shadow fleet legally constitutes ‘non-innocent’ passage. Consequently, applying domestic criminal jurisdiction to foreign ships in transit remains largely untested.

Furthermore, recent intelligence indicates a shift in tactics, with shadow fleet vessels increasingly re-flagging and registering themselves as Russian. This manoeuvre aims to close the ‘stateless’ legal loophole, meaning the UK may not have grounds to interdict such a ship under UNCLOS Article 110. Interdicting a ship with a Russian flag, according to some commentary, could carry a heightened risk of direct diplomatic or military confrontation with Russia.