A Russia-linked tanker suspected of cutting Baltic Sea cables has been tugged to port amid reports it was loaded with “spy equipment”.

Finnish authorities boarded the Eagle S tanker at sea after suspecting the vessel of sabotaging undersea power and internet lines on Christmas Day.

The Cook Islands-registered ship was carrying Russian oil when it allegedly slowed down and dragged its anchor along the seabed to damage the Estlink 2 undersea cable, which provides power to Estonia.

A Finnish coastguard crew boarded the Eagle S on Thursday and sailed the vessel to Finnish waters, a coast guard official said.

The vessel is said to have been kitted out with special transmitting and receiving devices that monitor all naval activity, according to shipping journal Lloyd’s List citing a source with direct involvement in the ship.

“They were monitoring all Nato naval ships and aircraft,” the source said, adding: “They had all details on them. They were just matching their frequencies.”