Russian spy ship Yantar is operating on the edge of British waters and has directed lasers at pilots of surveillance aircraft monitoring its activities, the Defence Secretary has said.
John Healey warned Russian President Vladimir Putin, saying the UK "is ready" to respond as "military options" have been drawn up if the ship heads south.
The vessel is designed for gathering intelligence and mapping crucial undersea cables has loitered off the northern coast of Scotland.
It entered wider UK waters over the last few weeks.
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During a Downing Street press conference on Wednesday, Mr Healey said: "We deployed a Royal Navy frigate and RAF P-8 planes to monitor and track this vessel's every move, during which the Yantar directed lasers at our pilots".
"That Russian action is deeply dangerous. This is the second time this year that this ship, the Yantar, has deployed to UK waters," he added.
Mr Healey also said: "It is part of a Russian fleet designed to put and hold our undersea infrastructure and those of our allies at risk.
"It isn’t just a naval operation. It’s part of a Russian programme driven by what they call the Main Directorate of Deep-Sea Research, or GUGI, and this is designed to have capabilities which can undertake surveillance in peacetime and sabotage in conflict."
An image of the ship detected off the British coast
|PA
One of Putin's spy ships, the Yantar, has been detected in UK waters over the last few weeks
| PAThe Defence Ministry echoed Mr Healey's statement that it was "the second time this year that this Russian spy ship has deployed to UK waters".
"It is one of many Russian vessels designed to threaten our Critical Underwater Infrastructure," the MoD said on X.
"To Russia: We see you. We know what you are doing. And if Yantar travels South this week, our forces are ready to act."
In January the Yantar was caught lurking over undersea cables and was warned off by a Royal Navy nuclear-powered submarine.
Defence Secretary John Healey delivering a speech on Wednesday
|PA
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The UK and Nato allies are increasingly concerned about the risk Moscow poses to offshore cables, pipelines and other infrastructure which is critical to internet connectivity.
Any attack on undersea cables could cause "catastrophic disruption" to the financial and communications systems Britons rely on, the National Security Strategy Committee warned in September.
The Yantar has been within the UK's exclusive economic zone - it extends to 200 nautical miles - about 230 miles offshore.
It has has been on the edge of Britain's territorial waters within 12 nautical miles or 12.8 miles from the coast.
It is understood the laser incident occurred within the last fortnight.
Mr Healey said: “Clearly, anything that impedes, disrupts or puts at risk pilots in charge of British military planes is deeply dangerous".
“This is the first time we’ve had this action from Yantar directed against the British RAF," he added.
“We take it extremely seriously.
"I’ve changed the Navy’s rules of engagement so that we can follow more closely, monitor more closely, the activities of the Yantar when it’s in our wider waters."
The previous submarine response could be an option.
Others include warships following the example of the Dutch Navy, which escorted the Yantar out of the Netherlands' part of the North Sea earlier this month.
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