The Russia-linked tanker which damaged an undersea electricity cable on Christmas Day also dropped sensors in the English Channel, sources have said.

Finnish Special Forces seized the spy ship Eagle S, which operates under ‘dark fleet’, after it dragged its anchor to disrupt electricity supplies between Nato allies Finland and neighbouring Estonia.

It is the first time that a commercial ship suspected of sabotage has been taken into custody by authorities, in another huge blow to Vladimir Putin.

Sources have told maritime publication Lloyd’s List that the vessel – part of a shadow fleet used to bypass sanctions against Russia – was bristling with surveillance equipment along with laptops which would not normally be found on an oil tanker.

The equipment was carried in “huge portable suitcases” and sucked so much power from its generators that it led to repeated blackouts.

At least one non-crewmember was aboard. “They were monitoring all Nato naval ships and aircraft,” said the source.

“They had all the details on them. They were just matching their frequencies. Russians, Turkish, Indian radio officers were operating it.”

The crew on Eagle S would have been aware of its spying activities “as this could not be hidden” but were “threatened with their life, so everybody kept quiet”, the source added.

The intelligence gleaned would be taken to Russia to be analysed.

Eagle S also dropped “sensors-type devices” in the English Channel during its transit, they said.

And Eagle S was not alone, with sources confirming that another related tanker, Swiftsea Rider, is also carrying similar equipment.

The vessels are two of a batch of 26 ageing tankers linked to companies sanctioned by the UK government for “propping up Putin’s war machine”.

The damage to Estlink 2 – which won’t be fully repaired until July – is the third recent incident in the Baltic Sea.

Last month, the Arelion cable between the Swedish island of Gotland and Lithuania was damaged, as was the C-Lion 1 cable between the Finnish capital, Helsinki, and the German port of Rostock.

A Chinese ship belonging to the same batch of tankers , the Yi Peng 3, dragged its anchor over the cables in a separate act of sabotage.

While Finland has not invoked Nato’s Article 5 over what could be perceived as an act of war, Estonian Prime Minister Kristen Michal said that they may invoke Article 4, which involves consultation if any member state feels threatened.

Estonia’s foreign minister Ambassador Jonatan Vseviov posted on social media: ““We’re awake. We’re vigilant. We’re ready.And most importantly – we’re not alone. We’ve got this.”

The country then confirmed it will step up naval patrols in the Gulf of Finland.

Nato Secretary General Mark Rutte confirmed that Nato would also boost its presence in the Baltic.

Shadow Armed Forces Minister, Mark Francois, said: “The English Channel is the busiest shipping lane in the world, so the suggestion a Russian ‘shadow fleet’ vessel may have been dropping devices there, whilst in transit, is very concerning.

“All the more credit then, to our new Nato allies in Finland for boarding and searching the ship, when she was subsequently acting suspiciously in the Baltic. Clearly, the Finns are going to be a force to be reckoned with, as part of the Nato Alliance.”

Former defence minister Tobias Ellwood said: “It’s not only plausible but very probable that the Eagle S dropped sensors as it passed through the English Channel.

“The question is – how many other Russian merchant vessels are conducting similar operations as part of Moscow’s Grey Zone against the West? And what are we doing about it? “