UK Shadow Fleet Interdiction: Legal Basis and the Yantar Problem

UK DEFENCE FORUM INTELLIGENCE NOTE

The Announcement
On 25 March 2026, Prime Minister Starmer announced at the Joint Expeditionary Force summit in Helsinki that the United Kingdom would begin interdicting vessels of the Russian shadow fleet operating in British waters. The announcement was timed to coincide with commitments from other JEF member states to coordinate enforcement across the North Sea, Baltic and Norwegian Sea approaches. The English Channel, through which approximately 544 dark fleet vessels on the UK sanctions list regularly transit, was identified as the primary enforcement zone.

This note addresses two questions that the announcement raises but does not fully answer: what is the legal basis for interdiction, and could the same basis be applied to vessels such as the research ship Yantar -- Russia's primary seabed intelligence-gathering platform in British and Irish waters?

The Legal Basis for Shadow Fleet Interdiction

Pillar One: The Stateless Vessel Principle
The most robust legal foundation lies in UNCLOS Article 92, which establishes that ships must sail under the flag of one state only, and Article 110, which permits warships of any nation to board a vessel that is without nationality -- that is, stateless or falsely flagged. A vessel that has had its flag registration revoked, or that is sailing under a flag obtained fraudulently, enjoys no sovereign protection whatsoever.
This principle has already been tested in practice. When the tanker Jaguar -- previously registered to Gabon, which revoked its flag registration on the same day UK sanctions were issued -- was operating in the Baltic, Estonian warships boarded it under precisely this authority. The legal crack in the wall had been found. The UK's announcement of 25 March effectively commits Royal Navy and Border Force assets to applying the same principle systematically in British waters rather than case by case.

Pillar Two: The 2018 Sanctions Act
The Sanctions and Anti-Money Laundering Act 2018 provides the domestic legal underpinning. UK ministers have argued that it permits the boarding, detention and in extremis seizure of vessels operating under sanctions, particularly where those vessels are engaged in activity that breaches the sanctions regime -- carrying Russian crude oil above the G7 price cap being the primary trigger. The Act's enforcement provisions are supplemented by Maritime Enforcement Powers under the Policing and Crime Act 2017, which gives Border Force officers broad powers of boarding and search in British territorial waters and the contiguous zone.
The combination of the stateless vessel principle and the 2018 Sanctions Act gives the UK a credible legal basis for interdiction that has now survived its first operational tests in Estonia and in a French-led boarding in the Mediterranean on 20 March 2026. The first UK-waters operation is expected imminently.

Could This Apply to Yantar?
The short and legally unambiguous answer is no -- and understanding why matters as much as the interdiction announcement itself.
The Yantar is a commissioned vessel of the Russian Navy, operated by GUGI (the Main Directorate of Deep-Sea Research) under the Russian Ministry of Defence, homeported at Severomorsk with the Northern Fleet. Under UNCLOS Articles 29, 95 and 96, and under the long-established customary international law principle of sovereign immunity, warships and government vessels operated for non-commercial purposes enjoy complete immunity from boarding or seizure by any other state -- in international waters, in Exclusive Economic Zones, and with very narrow exceptions even in territorial seas.
This immunity holds regardless of what the vessel is doing. When the Irish Defence Forces confirmed that Yantar's extended presence in their Exclusive Economic Zone was 'in line with UNCLOS rules for transit through international waters,' they were not being naive. They were accurately stating the legal position. The Yantar can loiter for weeks over Microsoft and Google subsea cables in the Irish Sea, operate with its AIS transponder off, direct lasers at RAF patrol aircraft, and shadow Norwegian research vessels -- and none of it is actionable under current international law.
Russia has structured its seabed intelligence programme with deliberate legal precision around this immunity. GUGI research ships fly the Russian naval ensign and maintain their government vessel status specifically to ensure this protection. The deniable damage work -- dragging anchors over cables -- is then contracted out to third-country-flagged commercial vessels (the Eagle S, the NewNew Polar Bear, the Yi Peng 3) which are legally distinguishable from Yantar in every respect that matters for interdiction purposes.

The Remaining Gap
The one potential legal hook that maritime law analysts have identified is SOLAS -- the International Convention for the Safety of Life at Sea -- which requires vessels to submit to safety inspections. However, SOLAS Regulation 1 explicitly exempts warships and government non-commercial vessels. A vessel could theoretically be required to enter port if a specific safety violation were identified, but this has never been tested against a Russian naval vessel and would almost certainly trigger a diplomatic confrontation disproportionate to the intelligence gain.
The gap between what Western governments can see Yantar doing and what they can legally do about it is therefore a structural feature of the current international legal framework -- not a failure of political will. Closing it would require either a revision of UNCLOS provisions on sovereign immunity (not practically achievable) or the development of new multilateral legal instruments specifically targeting state-directed seabed interference -- a long-term project that the EU's February 2025 Cable Security Action Plan begins to address.
In the interim, the practical response is what the UK has now announced: maximum legal pressure on the deniable commercial component of Russia's seabed warfare programme, combined with persistent military shadowing of Yantar and its sister vessels -- making clear that their activities are observed, recorded, and will be used as evidence in any future legal or diplomatic proceedings -- while accepting that direct interdiction of a Russian naval vessel remains beyond the current legal framework.

Robin Ashby Director General U K Defence Forum March 2026