UK forces helped the French navy intercept an oil tanker under international sanctions that was coming from Russia, alleged to be part of its shadow fleet.
French President Emmanuel Macron announced the capture in a social media post on Monday, adding that it happened with support from several allies, including Britain.
The ship, called Tagor, was boarded in international waters in the Atlantic on Sunday, Mr Macron said.
British forces on a helicopter from on board HMS Somerset provided monitoring and tracking support to the French navy before they boarded the tanker, the Ministry of Defence (MoD) said.
Mr Macron shared footage of a soldier abseiling from a helicopter onto the tanker and two armed soldiers marching around the deck.
French maritime authorities said the tanker captured on Sunday, which was travelling from the northwestern Russian port of Murmansk, was intercepted more than 400 nautical miles west of France.
It said the tanker was suspected of operating under a false flag and that the French navy was now escorting it to an anchorage for more checks.
Russia is believed to be using a fleet of vessels - often referred to as its shadow fleet - to carry oil and goods around the world to avoid sanctions on Moscow following its full-scale invasion of Ukraine.
Oil revenue is a key part of Russia's economy, allowing Vladimir Putin to pour money into his war effort in Ukraine without worsening inflation for residents and avoiding a currency collapse.
The UK and France have vowed to prevent Russian-linked ships passing through their waters.
"Alongside our allies, we are stepping up our response to shadow vessels - to choke off the funds that fuel Putin's illegal invasion of Ukraine," an MoD spokesperson said.
'UK challenged 700 suspected shadow fleet vessels'
Meanwhile, Downing Street rejected suggestions the UK did not have the military capability to lead an operation aimed at seizing a shadow fleet tanker.
Sir Keir Starmer's official spokesman said: "The UK is disrupting and deterring shadow fleet vessels and their harmful maritime activity, and since October 2024 we have challenged more than 700 suspected shadow fleet vessels.
"I'm not going to comment on specific operational planning or give a running commentary, as this could compromise our ability to successfully take action against these ships and only benefit our adversaries.
"But as we have said previously, any target ship will be individually considered by law enforcement, military, and energy market specialists before an operation is executed."
Mr Macron added that is was "unacceptable that boats skirt international sanctions, violate the law of the sea and finance the war that Russia has been waging for more than four years against Ukraine".
"These ships, that don't respect the most elementary rules of maritime navigation, are also a threat to the environment and everyone's security," he added.
Latest in a series of oil tanker seizures
In March, the UK helped the French navy seize the Deyna in the Mediterranean.
The oil tanker is a Mozambique-flagged vessel which is sanctioned by the UK and the European Union for its role in shipping Russian oil.
The same month Mr Starmer permitted British forces to seize shadow fleet vessels, leading the ships to veer way from the English Channel.
British forces helped the US seize the oil tanker Marinera in the Atlantic and France intercept another sanctioned ship, the Grinch, in the western Mediterranean in January. The Grinch was released in February after paying a multimillion-euro penalty.
Read more:
UK and France seize Russian tanker in Med
Analysis: Dark ships and shadow fleets
While European sanctions remain in place, the US has temporarily eased sanctions on the sale of Russian oil as the Middle East war disrupts oil supplies and boosts prices.
Moscow has previously called the seizure of its tankers, or vessels carrying its cargoes, an act of piracy.
Kremlin spokesman Dmitry Peskov reiterated this on Monday and said that Russia "considers such actions illegal", when asked about the French-led action.
(c) Sky News 2026: Britain helps the French navy seize Russian 'shadow fleet' tanker in the Atlantic
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