The UK has seized its first superyacht in British waters as part of sanctions against Russia.
The £38m yacht, named Phi, is owned by an unnamed Russian businessman.
Transport Secretary Grant Shapps said the individual was not currently sanctioned but had “close connections” to Russian President Vladimir Putin.
The UK has introduced a raft of sanctions against Russian individuals and businesses following the country’s invasion of Ukraine.
The 58.5m (192ft) Phi was first identified as being potentially Russian-owned on 13 March but its ownership is “deliberately well hidden”, the government said.
It added that the company the ship is registered to is based in the Caribbean islands of St Kitts and Nevis but it carried Maltese flags to hide its origins.
The Department for Transport (DfT) would not comment on why it was not naming the individual who owns the yacht.
Transport secretary Mr Shapps said the move was “a clear and stark warning to Putin and his cronies”.
“The ship won’t be going anywhere for the time being,” he said. “People who have benefitted from [Mr Putin’s] regime cannot benefit from sailing around London and the UK in ships like this.”
On its website, the ship’s builder Royal Huisman describes Phi – which is named after the mathematical concept also known as the Golden Ratio – as “magnificently sensuous”.
The bright blue yacht features what the builders call an “infinite wine cellar”, as well as a fresh-water swimming pool and penthouse apartment on the upper deck.
The ship, which was built in the Netherlands, made her maiden voyage last year.
UK officials boarded Phi in Canary Wharf, east London on Tuesday. The vessel was in the capital for a superyacht awards ceremony and was due to depart at 12:00.
The yacht was detained under the Russia (Sanctions) (EU Exit) Regulations 2019. The regulations say the secretary of state “may give a ‘movement direction’ to any ‘ship owned, controlled, chartered or operated by persons connected with Russia’,” according to Benjamin Maltby, partner at international law firm Keystone Law.
Yachts count as ships and a movement direction could include detaining the ship, he said.
However, he said the decision to detain a ship could be challenged under human rights law, which gives people the right to “peaceful enjoyment” of their possessions.
A successful challenge could see the owner demanding compensation, starting at the cost of chartering a similar yacht, which would be around £250,000 a week, Mr Maltby said.
“If the secretary of state has got this wrong, it could be a very expensive mistake,” he added.