The Russian spy ship Yantar has used lasers for the first time to disrupt RAF pilots tracking its activity near UK waters, the defence secretary has said.
John Healey said in a Downing Street news conference that the "deeply dangerous" move from the Yantar was being taken "extremely seriously" by the government.
He added that the vessel was north of Scotland and had entered UK waters for the second time this year during the last few weeks.
The UK would continue monitoring the ship and had "military options ready should the Yantar change course," he added.
"My message to Russia and to Putin is this: we see you. We know what you're doing. And if the Yantar travels south this week, we are ready," he added.
Healey said the laser incident took place whilst the Yantar was being followed by a Royal Navy frigate and RAF Poseidon P-8 planes deployed to "track the vessel's every move". It is understood the episode occurred within the last two weeks.
Speaking at the news conference, the defence secretary added he had changed the Royal Navy's rules of engagement so that it could follow the Yantar more closely "when it is in our wider waters".
Healey said the vessel, which entered service in 2015, was part of Russia's Main Directorate for Deep Sea Research (GUGI), which was designed to "undertake surveillance in peacetime and sabotage in conflict".
"We have military options ready should the Yantar change course. I am not going to reveal those because that only makes President Putin wiser," he said.
Asked about the risks posed by lasers, Healey replied: "Anything that impedes, disrupts or puts at risk pilots in charge of British military planes is deeply dangerous."
On 6 November, the Dutch Navy said two of its vessels escorted the Yantar out of the North Sea, where it was operating near Dutch territorial waters.
While the Yantar's current position is not clear, flight tracking website FlightRadar24 shows an RAF Poseidon P-8 surveillance plane circling off the Scottish coast, although it is not certain that this is tracking the Yantar.

The MOD released new photos on Wednesday of the Royal Navy tracking the Yantar
Russia describes Yantar, which is operated by the country's Ministry of Defence, as an oceanic research vessel. Western nations have often tracked it in European waters and they suspect part of its mission has been to map undersea cables.
The Russian Embassy said in a statement, it was "not interested in British underwater communications".
"The actions of our country do not touch interests of the UK and they are not aimed at undermining its security," the statement said.
It added: "However, London, with its Russophobic path and increasing militaristic hysteria leads to further degradation European security, providing the premise for new dangerous situations.
"We call on the British side to hold off taking any destructive steps which might aggravate the crisis situation on the European continent."
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