National Crime Agency (NCA) officers have detained an Indian national on suspicion of sanctions violations following the dramatic interception of a Russian oil tanker in the English Channel on Sunday.
The individual was taken into custody for questioning by investigators after an operation conducted by Royal Marine Commandos alongside the NCA.
The NCA confirmed that 24 crew members, who are Georgian and Indian nationals, remained on board the Smyrtos, which is anchored off the Dorset coast.
Sunday's operation, during which commandos descended from a helicopter onto the tanker via fast-roping, marked the first such action carried out by UK armed forces.
Russia relies on hundreds of oil tankers that have been sanctioned by the UK and other Western nations due to its invasion of Ukraine.
British sanctions prohibit vessels from this so-called "shadow fleet" from entering UK ports and also bar British companies and individuals from offering financial, insurance, or brokerage services to ships that supply or transport Russian oil.
The Ministry of Defence (MoD) reports that the UK has imposed sanctions on over 500 such vessels.
Prime Minister Sir Keir Starmer announced in March that British armed forces were "now able to board sanctioned vessels that are passing through our waters".
On Sunday, he stated: "This successful operation delivers yet another blow to Russia and reminds those fuelling [Russian President Vladimir] Putin's war in Ukraine that we will not let them hide."
Ukrainian President Volodymyr Zelensky expressed gratitude to the UK for taking an "important step against Russia's oil fleet".
There was no immediate reaction from Moscow, which has previously denounced similar interceptions as unlawful and "bordering on international piracy".
According to tracking website MarineTraffic, the Smyrtos operates under a Cameroon flag.
BBC Verify has determined that the vessel began its journey on 5 June from Russia's Ust-Luga port, an oil terminal near St Petersburg, before crossing westward into the channel on Saturday.
The ship was sanctioned in July 2025 and has since changed its name from Myrtos to Smyrtos, as well as altering its flag twice.
The MoD stated that the operation took place in international waters, as it was more than 12 nautical miles from the UK coast.
It emphasised that the action was fully compliant with both domestic and international law.
The interception was the outcome of weeks of military and political planning, according to an MoD spokesperson.
It was carried out in close coordination with the French, the MoD added, 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 belonging to the "shadow fleet" with UK assistance.
A British helicopter provided support during that operation, the MoD informed the BBC.