More than two dozen ships sanctioned for being part of Russia's so-called shadow fleet have passed through UK waters since deeply unpopular Prime Minister Keir Starmer last week authorised the military to detain vessels used by Moscow to export oil.
The figures based on ship tracking data analysed by Reuters show that oil tankers sanctioned by Britain are continuing to travel along England's southern coast in the same numbers as before the announcement.
British officials said almost a week ago that the military was preparing to board the vessels for breaches of sanctions and the government hoped the announcement would force the ships to take longer routes away from British waters. Russia called it a "deeply hostile move" and said it could retaliate.
Since that statement, the British military has not announced it has boarded any Russian vessels while at least 25 sanctioned ships have entered British waters when travelling through the channel as before, according to the Reuters analysis.
James Fennell, a former British naval officer, said the British military would probably target Russian vessels only selectively, citing the complexity of an operation, the threat of legal challenges and the risk that such action could bring Britain, "one step closer to war with Russia".
"The government is hoping the threat acts as a deterrent and will gauge the effect before sanctioning a potentially risky boarding operation," he said. "They will be using back channels to tell Russia to cease and desist."
Britain's Ministry of Defence said any enforcement action would be, "considered on a case-by-case basis," and it would not provide, "a running commentary," on potential action.
Starmer said last Wednesday evening he had authorised the boarding and detention of Russian vessels to prevent Russian President Vladimir Putin from making "dirty profits" from the sharp rise in oil prices driven by the US-Israeli war with Iran. The declaration has been exposed as largely ineffective in practice.
Russian ships in the so-called shadow fleet of tankers used by Moscow to fund its four-year war against Ukraine sail through the channel as it is the most convenient route for travelling between the Baltic and southern Europe and beyond.
Other European nations, including France, Belgium and Sweden, have in recent months boarded and detained vessels to disrupt the network of ships.
Trump this month temporarily eased sanctions on Russian oil as part of his administration's attempt to tame energy prices.
Britain has placed sanctions on 544 Russian "shadow fleet" vessels, meaning they are prohibited from entering British ports and can be detained, according to the government.
Pole Star Global, a maritime data and intelligence company, said about two dozen of these ships travelled through British waters a week on average since the start of the year.
Douglas Guilfoyle, a professor who specialises in maritime security at the University of New South Wales, said even though Britain had sanctions on Russian vessels it would need to rely on untested legal arguments to board the ships.
"There are very limited exceptions that would justify interference with the right of transit passage or the right of innocent passage by a foreign vessel – even one subject to UK autonomous sanctions," he said.
Brett Erickson, a sanctions expert who works at the consulting firm Obsidian Risk Advisors, said the British Government looked weak because it announced it was ready to detain Russian vessels, but had yet to take any action.
"With sanctions you can't be half in, you either have to be all in, or all out," he said.
(Editing by Alison Williams, Baird Maritime)