National Crime Agency (NCA) officers have arrested an Indian national on suspicion of sanctions offences after the dramatic seizure of a Russian oil tanker in the English Channel on Sunday.
He was taken into custody for questioning by investigators following the operation mounted by Royal Marine Commandos along with the NCA.
The NCA said 24 Georgian and Indian crew members remained aboard the Smyrtos, anchored off the Dorset coast.
Sunday's operation, which saw commandos fast-roping from a helicopter on to the tanker, was the first of its kind carried out by UK armed forces.
Russia uses hundreds of oil tankers sanctioned by the UK and other Western states over its invasion of Ukraine.
British sanctions ban vessels from this "shadow fleet", as it is known in the West, 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.
The Ministry of Defence (MoD) says the UK has sanctioned more than 500 such ships.
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 said: "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 thanked the UK for taking an "important step against Russia's oil fleet".
There was no immediate response from Moscow, which has previously described similar interceptions as illegal and "bordering on international piracy".
According to tracking website MarineTraffic, Smyrtos sails under a Cameroon flag.
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 MoD said the operation had happened in international waters as it was more than 12 nautical miles from the UK coast.
It said it had been in full compliance with both domestic and international law.
The interception was the result of weeks of military and political planning, an MoD spokesman said.
It 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 the "shadow fleet" with UK support.
A British helicopter provided support during that operation, the MoD told the BBC.
