"British military will be able to board shadow fleet vessels transiting UK waters as the UK steps up its pressure" on Russian President Vladimir Putin, Downing Street said in a statement.

London has sanctioned hundreds of vessels suspected of being part of the shadow fleet used by Russia to bypass Western embargoes since the invasion of Ukraine in 2022.

The ships -- usually ageing tankers with dubious ownership -- are banned from accessing UK ports and services.

"The prime minister has agreed that the UK Armed Forces and law enforcement officers will now be able to interdict vessels that have been sanctioned by the UK and are transiting through UK waters," Downing Street said.

"The UK will put a further stranglehold on the shadow fleet, closing off UK waters, including the Channel, for sanctioned vessels. This will in turn force operators to either divert to longer, financially painful routes, or risk being detained by British forces."

"Putin is rubbing his hands at the war in the Middle East because he thinks higher oil prices will let him line his pockets," said UK Prime Minister Keir Starmer.

"That's why we're going after his shadow fleet even harder, not just keeping Britain safe but starving Putin's war machine of the dirty profits that fund his barbaric campaign in Ukraine."

Starmer is due to address a summit of the Joint Expeditionary Force -- a UK-led defence alliance of 10 member countries -- in Helsinki on Thursday.

The move comes as Washington eased restrictions on Russian oil to soften prices sent soaring by the US-Israel war against Iran.

In January, London provided support to Washington during a US operation to seize a Russia-linked oil tanker called the Marinera -- previously known as the Bella-1 and accused of flying a false flag.

France, Belgium, Finland and other European countries have also recently seized sanction-busting vessels believed to belong to Russia's ghost fleet.

"Following the detention of a ship, criminal proceedings may be brought against the against the owners, operators and crew, for breaches of UK sanctions legislation," Starmer's office said.

Each operation under the new approach will be considered by law enforcement and require ministerial sign-off, according to the government.

The UK government says it has sanctioned 544 vessels thought to be part of the shadow fleet -- which it claims transports around 75 percent of Russia's crude.