The Russian ship was seen on the edge of UK waters on Wednesday. The ship can map undersea communication cables.
Defence Secretary John Healey warned Russia’s President Putin in response.
Healey said the vessel had been firing lasers at British pilots sent to monitor it, and confirmed this is the second time this year the ship has been deployed to British seas.
The defence secretary warned Vladimir Putin: "We see you, we know what you’re doing, and if the Yantar travels south this week, we are ready."
He also said the ship "is part of a Russian fleet designed to put and hold our undersea infrastructure and those of our allies at risk".
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