The government has identified a legal basis which it believes can be used to allow UK military to board and detain vessels in so-called shadow fleets, BBC News understands.
Russia, Iran and Venezuela have all been accused of operating ships without a valid national flag to avoid sanctions on oil.
It is understood there are plans for the armed forces to use these powers, in what is being described inside government as a ramping up of action against the ships.
It is not known exactly when the first UK military action might occur.
Two oil tankers subject to US sanctions were reportedly spotted sailing east through the English Channel towards Russia on Thursday.
The UK has already imposed sanctions on more than 500 alleged shadow vessels, which it believes are helping to fund hostile activity, including Russia’s invasion of Ukraine.
Ministers say that action taken by the UK and its allies has forced around 200 ships off the seas, most of which will have been operating without a legitimate flag.
Ship flagging is the process used to register a vessel to a specific country, which then allows it to travel in international waters and offers it certain protections under law.
The government believes the new legal mechanism they have identified could be applied to any sanctioned vessels not legitimately flagged.
Officials say this would have included the Marinera tanker, which was seized last week.
The Marinera, a Venezuelan-linked ship previously known as the Bella 1, was stopped by the US Coast Guard as it travelled through the North Atlantic ocean between Iceland and Scotland.
The Ministry of Defence said the US asked the UK for assistance, and that RAF surveillance aircraft and a Royal Navy support ship RFA Tideforce took part in the operation.
Defence Secretary John Healey said the action was “in full compliance with international law”, adding the UK “will not stand by as malign activity increases on the high seas”.
Healey told MPs on Wednesday that the government was “stepping up action on the shadow fleet, developing further military options and strengthening co-ordination with allies”.
It is understood that identifying this legal mechanism was one of the further military options that Healey was referring to.
A Ministry of Defence spokesperson said: “The defence secretary set out in parliament this week that deterring, disrupting and degrading the Russian shadow fleet is a priority for this government”.
They added: “We will not comment on specific operational planning”.
Transport Secretary Heidi Alexander said on Sunday that the number of insurance checks were being increased, with more than 600 ships stopped while sailing close to the British Isles.
Vessels not legitimately flagged generally have no insurance, which experts have warned could lead to a crisis if they were involved in an incident like an expensive oil spill.
But Alexander said it would not be appropriate to say how many alleged shadow vessels were known to have sailed in UK waters.
She told Sky News: “Providing you with that information only helps one person and that is President Putin.”
Out on the western Baltic, a coastguard officer radios a nearby, sanctioned oil tanker.
“Swedish Coastguard calling… Do you consent to answer a few questions for us? Over.”
Through heavy static, barely audible answers crackle over from a crew member, who gradually lists the ship’s insurance details, flag state and last port of call – Suez, Egypt.
“I think this ship will go up to Russia and get oil,” says Swedish investigator, Jonatan Tholin.
This is the front line of Europe’s uneasy standoff with Russia’s so-called “shadow fleet”; a term that commonly refers to hundreds of tankers used to bypass a price cap on Russian oil exports.
After the Kremlin launched its full-scale invasion of Ukraine, many Western countries imposed sanctions on Russian energy, which Moscow is accused of dodging by shipping oil on aged tankers often with obscure ownership or insurance.
European coastguards and navies are regularly coming in contact with vessels in Russia’s shadow fleet which they suspect are transporting oil [BBC]
Some “shadow” ships are even suspected of undersea sabotage, illicit drone launches or “spoofing” their location data.
Out on the waves, where freedom of navigation is a golden rule, the ability and appetite of coastal countries to intervene is limited, even though the risk they face is escalating.
As the BBC has learned, a growing network of “shadow” ships are sailing without a valid national flag, which can render vessels stateless and without proper insurance.
That is a troubling trend, given many are practically “floating rust buckets”, says senior maritime intelligence analyst at Windward AI, Michelle Wiese Bockmann. If there is an accident, like a billion-dollar oil spill, “good luck with trying to find somebody responsible to pick up any cost”.
Driven by record sanctions and tighter enforcement, the number of falsely flagged ships globally has more than doubled this year to over 450, most of them tankers, according to the International Maritime Organization (IMO) database.
The BBC has been tracking one ship that appears to have sailed without a valid flag.
The head of Estonia’s navy, Commodore Ivo Värk, says that vessels travel back and forth, past Estonia, to major Russian oil terminals [BBC]
The head of Estonia’s navy, Commodore Ivo Värk, says they have seen dozens of such passing vessels this year whereas they used to see just one or two.
The rise is alarming, he tells me, as we talk in his office overlooking the Gulf of Finland, a narrow gateway to the major Russian oil terminals of Ust-Luga and Primorsk.
What’s more, he suggests, it’s brazen: “There’s no secret made about it.”
We spot the tanker Unity on the MarineTraffic app, the day we board an Estonian (British-built) Minehunter that is also used in Nato’s Baltic Sentry patrols to protect critical infrastructure.
Journeying east, Unity is over 100 miles away but sailing in our direction.
The BBC has investigated its history and it offers an illuminating insight into the enigmatic life of a shadow ship.
Tracking data shows that Unity has passed through the English Channel four times in the last twelve months, including journeys between Russian ports and India; a key oil customer that has not signed up to the price cap.
Originally known as Ocean Explorer, the tanker was built in 2009 and flew the flag of Singapore for more than a decade.
By late 2021, the vessel – which that year operated under the name Ocean Vela – took the flag of the Marshall Islands but was struck from that list in 2024, a registry spokesperson told us, because the ship’s then-operator and beneficial owning company had been sanctioned by the UK.
The tanker appears to have had three further names since 2021 (Beks Swan, March and Unity) and three further flags (Panama, Russia and Gambia) but always retains a unique IMO number.
In August, ship broadcasting data shows Unity claimed the flag of Lesotho which was designated as “false”. Lesotho is a small, landlocked African enclave kingdom that, according to the IMO, does not have an official registry.
Over the past year, Unity has made several voyages to and from Russian oil ports [BBC]
The BBC has tried to contact Unity’s listed owner, a Dubai-registered company called FMTC Ship Charter LLC, but our emails and our calls went unanswered.
The beneficial owners of 60% of shadow fleet vessels remain essentially unknown, according to maritime intelligence company Windward AI.
Opaque ownership structures – and frequent name or flag changes – have become a signature trait of the shadow fleet as a means of avoiding detection.
Purged from reputable registries and having circled the drain of poor-quality alternatives, some ships are now at a point “where they just don’t even bother at all”, says Michelle Wiese Bockmann.
Unity’s most recent journey saw it sail through the North Sea in late October before entering the Baltic and passing countries including Sweden and Estonia – the point at which we spotted it.
By 6 November, it was anchored outside the Russian port of Ust-Luga where it remains at the time of publication.
The tanker was added to the UK and EU’s ever-growing list of sanctioned vessels earlier this year but, like so many others, continues to do business despite other difficulties.
Back in January, it reportedly sheltered in the English Channel after suffering a mechanical failure during a storm. The following August it was reportedly detained at a Russian port due to technical issues and unpaid wages.
At the time of publication the vessel was anchored just outside the major Russian oil port, Ust-Luga [Planet Labs]
Unity is just one of hundreds of vessels subject to UK and EU service and port bans as both London and Brussels try to increase pressure on the Kremlin.
Nevertheless, Russian revenues from crude and oil product sales were $13.1bn (£9.95bn) in October alone, according to the Paris-based International Energy Agency (IEA) – although this was down by $2.3bn when compared with the same month a year ago.
Analysis by the Centre for Research on Energy and Clean Air finds that “shadow” tankers, either sanctioned or suspected, account for 62% of shipped Russian crude oil exports, while China and India are by far the biggest customers for crude, followed by Turkey and the European Union itself.
While politicians talk of toughening action, navy and coastguard officers point out that a country’s power to act fades the further you go out to sea.
The right of innocent passage remains a cornerstone of maritime law, but stateless vessels technically are not entitled to it.
Countries such as France, Finland and Estonia have detained ships, and they can do so where a crime is suspected, however such drastic controls remain a relatively rare event.
“There’s a complexity associated with it,” argues Commodore Ivo Värk. “With the Russian presence next to our borders, the risk of escalation is too high to do it on a regular basis.”
The Estonians speak from experience.
When they attempted to intercept a flagless tanker in May, Russia briefly deployed a fighter jet and has “constantly” had about two naval vessels in the Gulf of Finland since, says Commodore Värk.
The fear of escalation sits alongside broader concerns of commercial retaliation if a more aggressive approach were to be taken.
“Every day in the Baltic, there’s suspicious activity,” a Nato official told the BBC, speaking on the condition of anonymity. Nevertheless, the official added, “we don’t want to be cowboys and jumping on ships. The act of monitoring ships is a deterrent in itself”.
“Freedom of navigation is the lifeblood of all of our economies.”
Back on the bridge of the Swedish coastguard ship, the radio call with the sanctioned tanker has wrapped up.
“Thank you for your co-operation,” says the officer as the vessel carries on towards Russia.
The exchange lasted just over five minutes.
“You need to see it in a larger perspective,” says investigator Jonatan Tholin when I suggest these measures appear less than muscular: “This information can be used in our maritime surveillance.”
But as Europe steps up checks and watches the waves, Windward’s Michelle Wiese Bockmann spies something else: “You can literally see the international rules-based order crumbling through the sanctions-circumventing tactics of these vessels.”
There is a lot at stake for the environment and on security, she says, and meanwhile “the dark fleet is getting darker”.
The BBC approached Russia’s embassy in London for comment. In response, a spokesman said that the West’s “anti-Russian sanctions” were “illegitimate” and “undermine established principles of global commerce”.
“Labelling ships used to export Russian oil as ‘shadow fleet’ is discriminatory and misleading,” the embassy said, and instances of invalid flags were typically down to “easily resolved” issues such as administrative delays.
It was sanctioning countries, the spokesperson said, that had “heightened” the risks by “forcing shipowners and operators to navigate an increasingly fragmented and restrictive regulatory landscape”.
Additional reporting by Adrienne Murray, Michael Steininger and Ali Zaidi
The so-called Russian “shadow fleet,” used to circumvent international oil sanctions linked to the war in Ukraine, is raising increasing concerns within the European Union.
According to a working paper prepared for a meeting of EU foreign ministers on Monday in Luxembourg, the fleet is now estimated to include between 600 and 1,400 tankers.
Experts advising EU foreign affairs chief Kaja Kallas said the vessels not only contribute to the Russian war economy but also create major risks for the environment and maritime safety, according to a document seen by dpa.
Accidents involving these vessels could trigger oil spills and other marine pollution, with devastating effects on coastal areas, ecosystems and fisheries.
Many of the ships have opaque ownership structures and are either uninsured or underinsured. In such cases, experts note, taxpayers of coastal states could be left to cover the costs of any environmental damage.
Moscow has been relying on the shadow fleet for years to bypass a Western-imposed price cap on Russian oil. This often involves chartering older vessels with difficult-to-trace ownership and employing tactics to conceal the origin of oil cargoes.
Some ships have had their satellite-based automatic identification system (AIS) transponders switched off or manipulated, while others have transferred oil between tankers on the high seas.
Ships as a platform for drone attacks
Experts also warn that shadow fleet ships could be used as platforms for launching drones for jamming or espionage attacks. This concern grew following mysterious drone sightings in Denmark, which temporarily disrupted air traffic over the Copenhagen area.
To tackle the problem, Kallas is proposing stronger action against the vessels and shipping companies involved, alongside closer cooperation with coastal states and countries under whose flags the ships operate. Ideally, such states would consent to inspections by EU naval forces.
Since June 2025, three EU naval missions have been tasked with monitoring the shadow fleet and gathering intelligence, including Operation Aspides, which primarily aims to protect merchant vessels in the Red Sea from attacks by the Yemeni Houthi militia.
More than 560 ships on sanctions list
Looking ahead, the EU plans to expand its sanctions list. The number of ships facing port bans and punitive measures, pending approval from EU member states, is set to rise from 444 to 562 under the 19th EU-Russia sanctions package, according to the document.
French soldiers have boarded an oil tanker believed to be part of Russia’s “shadow fleet”, used to evade sanctions imposed because of the war in Ukraine.
The Boracay left Russia last month and was off the coast of Denmark when unidentified drones forced the temporary closure of several airports last week. It has been anchored off western France for a few days.
President Emmanuel Macron said the crew had committed “serious offences” at an EU leaders’ summit in Copenhagen on Wednesday but did not elaborate.
Kremlin spokesman Dmitry Peskov said Russia had no knowledge of the vessel.
AFP news agency quoted a source as saying French military personnel had boarded the vessel on Saturday.
Macron refused to be drawn on the question of whether the ship may have been used as a platform for the drone flights that caused such disruption in Denmark last week.
Prosecutors in Brest have opened an investigation on two counts: refusing an order to stop and failing to justify the nationality of the ship’s flag.
Many Western countries imposed sanctions on Russian energy by limiting imports and capping the price of its oil following Russia’s fill-scale invasion of Ukraine in 2022.
To evade these sanctions, Moscow built up what has been referred to as a “shadow fleet” of tankers whose ownership and movements could be obscured.
Russia is believed to have a fleet of several hundred tankers which are registered in other countries and are used to export its petrol. Macron said that Russia’s shadow fleet contained between 600 and 1,000 ships.
The Boracay, also known as Pushpa and Kiwala, is a Benin-flagged vessel but has been listed under UK and EU sanctions on Russia.
It was detained by Estonian authorities earlier this year for sailing without a valid country flag.
It had set off the Russian port of Primorsk outside Saint Petersburg on 20 September and sailed through the Baltic Sea and past Denmark before entering the North Sea and carrying on through the English Channel.
It had been scheduled to arrive in Vadinar in north-western India on 20 October, according to data from the Marine Traffic tracking website. However it was followed by a French warship after it rounded the Brittany coast and then altered course and headed east towards the French coast.
EU leaders have been meeting in Copenhagen under pressure to boost European defence after a series of Russian incursions into EU airspace, and days after drones targeted Danish airports.
Copenhagen airport, followed by several Danish airports and military sites on the Jutland peninsula, faced drone disruption last week.
Danish Prime Minister Mette Frederiksen told reporters that “from a European perspective there is only one country… willing to threaten us and that is Russia, and therefore we need a very strong answer back”.
Danish police have not found any evidence that Russia was behind last week’s drone disruption, but Frederiksen linked it explicitly to other hybrid attacks such as Russian drones over Poland.
It was part of a pattern that had to be viewed from a European perspective, she told reporters on Wednesday.
The incursions have become most acute for countries on the EU’s eastern flank such as Poland and Estonia.
A number of member states have already backed plans for a multi-layered “drone wall” to quickly detect, then track and destroy Russian drones.
