Dozens of oil tankers sanctioned by the UK in response to Russia’s war in Ukraine have sailed through the English Channel this month despite defence officials vowing to take “assertive action”.

Since its full-scale invasion of Ukraine, Russia has used hundreds of ageing tankers with obscured ownership – known as the shadow fleet – to evade sanctions on its oil exports.

The BBC understands that the UK government received legal assurances earlier this month that such vessels can be detained. Yet 42 sanctioned tankers tracked by BBC Verify passing through the English Channel did so after that advice was received.

Among them was the Sofos – a tanker sanctioned by the UK Foreign Office in May 2025. It moved through the Channel after travelling from Venezuela and is now near the Russian city of St Petersburg.

Ship-tracking data shows the Sofos loaded oil in Russia in mid-November, before travelling to Turkey and then to Venezuela, where it switched off its tracking signal. Satellite imagery later placed it at Venezuela’s Jose oil terminal on 22 and 23 December, before its signal reappeared outside the country’s waters on 26 December.

The Nasledie, a tanker which is more than 20 years old, also entered the Channel in January. The ship was sanctioned by the UK in May 2025 and according to Anna Zhminko, an analyst at Maritime Intelligence firm Vortexa, has been part of the shadow fleet since 2023.

In November the ship had a makeover, changing its name from Blint and switching to a Russian registry after falsely flying under the Comoros flag. It left Russia in late December and just days after the BBC reported on the UK’s legal advice it entered the Channel carrying about 100,000 tonnes of Urals crude, Russia’s flagship oil grade.

The shadow fleet has helped Russia to mitigate the impact of an embargo imposed on its oil exports since 2022 and has helped prop up an economy battered by Western sanctions.

In January, Foreign Secretary Yvette Cooper said the British government was ready to “tighten the chokehold” on Russia and take new “assertive actions” against shadow fleet vessels. Her comments came after officials were told troops could board and seize tankers under the Sanctions and Money Act 2018.

While the UK has aided US forces to seize a tanker near Iceland in early January and France in the Mediterranean on Thursday, British troops are yet to independently seize any shadow fleet vessels.

This is despite BBC Verify identifying six tankers operating in the Channel under false flags since the beginning of January. This means the ships are not registered in the states they publicly claim to be registered in. Under the United Nations Convention of the Law of the Sea such tankers can be seized as stateless vessels.

Dame Emily Thornberry, chair of Parliament’s Foreign Affairs Select Committee, said she was “very disappointed” that the vessels had not been intercepted.

“I think in order to give Ukraine proper support, we have to have not only sanctions that look on paper, but we have to make sure we mean it and that we implement those sanctions,” she said.

Mike Martin, a Liberal Democrat MP and former British Army officer, questioned why the UK has been reluctant to board the tankers independently, noting that the UK’s Royal Marines are trained and equipped for such operations.

“Its not that hard to project force 12 miles off the Straits of Dover,” he said. “It’s not that difficult to board a tanker.”

Russia also appears to have reacted to the UK’s vow to take more assertive action against the shadow fleet. On 20 January the Times reported that a warship escorted sanctioned tanker General Skobelev through the Channel.

And on 15 January, Russian foreign ministry spokeswoman Maria Zakharova said the Kremlin would view any detention of the tankers as a violation of international law which would be seen as “harming Russian interests”.

In a statement to BBC Verify, a Ministry of Defence spokesperson said: “Deterring, disrupting and degrading the Russian shadow fleet is a priority for this government. Alongside our allies, we are stepping up our response to shadow vessels – and we will continue to do so.”

Other tankers which have been sanctioned by the US and EU, but not the UK, have also sailed through the Channel over the course of January.

BBC Verify identified one vessel that took extreme measures to avoid detection and has been labelled by maritime experts as “the darkest of the dark fleet”.

The Arcusat – previously sanctioned by the US as the Tia, before changing its identity in 2024 – passed through the Channel on 8 January before being stopped by German federal police and turned around before it entered the country’s waters.

Last year, it obtained a new IMO number in a move that analysts said was almost unheard of. The unique code identifies ships and is supposed to be associated with a vessel for life, irrespective of changes of name or ownership.

Despite mounting calls for tougher enforcement, shadow fleet vessels remain difficult to monitor. Sanctioned tankers will deploy several strategies to evade detection – including flying under false flags, switching off their tracking signals, and broadcasting false locations.

“Once vessels start doing these direct port calls within Russia, they’re less afraid of compliance,” Zhminko told BBC Verify.

“Because it is already sanctioned it is easier for her to be a part of the open shadow fleet – she can’t do any mainstream market port calls because she is already sanctioned.”

According to the Center for Strategic and International Studies (CSIS), the shadow fleet tankers move between $87bn-$100bn worth of oil per year. Vaibhav Raghunandan, an analyst at the Centre for Research on Energy and Clean Air, said that 68% of all Russian crude oil was carried on sanctioned tankers in December 2025.

Mike Martin MP said that the revenue generated by the shadow fleet was helping to fund Russia’s war in Ukraine and providing the Kremlin with an economic lifeline. He said the conflict would only come to an end “when Russia’s economy collapses”.

“A key lever we have to pull is knock out oil sales through the shadow fleet,” Martin told BBC Verify. “So I think it’s massively important, it’s irritating we didn’t get on it much earlier.”

Additional reporting by Matt Murphy, Jack Fenwick and Joshua Cheetham.

Correction: The General Skobelev is the name of the sanctioned Russian tanker that was in the Channel, not the warship escorting it.