Royal Marine Commandos have boarded a Russian shadow fleet oil tanker in the English Channel in the early hours of Sunday morning.
Marines, joined by National Crime Agency officers, with the support of the RAF, intercepted and boarded the vessel in a six-hour operation - the first operation of its kind by UK armed forces.
The vessel, Smyrtos, will be held and monitored off the south coast of England as investigations continue, the MoD said.
Sir Keir Starmer said: "This successful operation delivers yet another blow to Russia and reminds those fuelling Putin's war in Ukraine that we will not let them hide."
In video footage shared by the MoD, armed personnel can be seen boarding the vessel by fast-roping from a helicopter.
Further videos show them conducting searches of cabins aboard the vessel, while officers from the NCA inspect paperwork.
Russia has been operating a "shadow fleet" of tankers to evade international sanctions imposed on its oil exports.
Responsible for carrying 75% of Russia's sanctioned oil, the shadow fleet of more than 700 vessels provides a critical lifeline for the Kremlin, the MoD said.
The prime minister announced in March that British armed forces were "now able to board sanctioned vessels that are passing through our waters".
The MoD said the UK has sanctioned more than 500 vessels.
The sanctions ban the vessels from entering UK ports and also prohibit British firms and individuals from providing financial, insurance, or brokerage services to ships that supply or deliver Russian oil.
According to tracking website MarineTraffic, Smyrtos sails under a Cameroon flag and is at anchor off the coast of Weymouth in the English Channel.
It began its journey on 5 June from Russia's Ust-Luga port, an oil terminal near St. Petersburg, before crossing west into the channel on Saturday, BBC Verify has found.
The vessel was sanctioned in July 2025 and has since changed its name from Myrtos to Smyrtos as well as the flag it sails under twice.
The interception comes after a week of resignations over the government's defence investment plan (DIP), which is set to be published before the Nato summit next month after months of delay.
John Healey resigned as defence secretary on Thursday, warning that the level of military spending proposed by Sir Keir "falls well short" of what is needed to protect the UK.
Al Carns also resigned as armed forces minister, telling the prime minister that the DIP was "neither transformative enough nor sufficiently funded".
Culture Secretary Lisa Nandy told the BBC the prime minister "had been clear" with his cabinet that they "had to find more money for defence", adding that discussions on the Defence Investment Plan were ongoing.
She told Sunday with Laura Kuenssberg the government had to "transform the way we do defence spending, so that what we're spending is fit for the threats we face now and in the future".
Shadow defence secretary James Cartlidge said the MoD needed as much as £28bn in extra funding over the next few years, adding that cutting welfare had to be "a big part of it".
The operation on Sunday was supported with aircraft from the Maritime Air Group, an RAF P-8 aircraft, as well as HMS Sutherland and HMS Ledbury.
Carns told the BBC the operation would have involved armed forces personnel "flying low level over the sea, rearing up before the ship, fast roping onto the ship, securing it and then taking it into our territorial waters".
He told Sunday with Laura Kuenssberg that now the UK had performed its first boarding of a Russian shadow fleet vehicle, "we're probably going to see more, should the opportunities present themselves".
Responding to the interception on Sunday morning, Attorney General Richard Hermer said: "This government made clear that we would pursue Russia's shadow fleet under the full force of international law."
The government has said it is targeting Russia's oil revenues to "choke off funding for Russia's war machine" in Ukraine.
Ukrainian President Volodymyr Zelensky thanked the UK for "taking this important step against Russia's oil fleet".
He wrote on X: "Europe urgently needs to take legislative steps to enable not only the detention of tankers and restrictions on oil shipments, but also the confiscation of the oil they carry."
Kemi Badenoch, the leader of the opposition, paid tribute to the "brave" military personnel, adding: "I support the Government in standing with Ukraine."
The operation was conducted in close co-ordination with the French, the MoD said, building on recent support provided by the UK to its allies.
French President Emmanuel Macron said on 1 June that his country's military had intercepted a sanctioned oil tanker suspected of being part of Russia's shadow fleet on Sunday with UK support.
A British helicopter provided support during the operation, the MoD told the BBC.
