The Royal Navy is sending a task force of seven ships to counter Russian activity off northern Europe, following "increasing concerns over security".
This major deployment will see six warships, a support ship and an RAF maritime patrol aircraft patrolling from the English Channel to the Baltic.
It is part of a Joint Expeditionary Force mission to protect critical undersea infrastructure such as communications cables and gas pipelines, according to the Ministry of Defence.
The force, made up of ten northern European nations, has agreed to mobilise in this way for the first time.
It comes as the nations are concerned about "an increase in attacks and threats" against undersea infrastructure, such as last year's attacks on the Nordstream gas pipelines.
Defence Secretary Grant Shapps said: "The UK and our JEF partners will do whatever it takes to defend our mutual areas of interest, and today’s display of unshakeable unity sends a powerful message of deterrence that we stand ready to meet any potential threat with force."
The Prime Minister Rishi Sunak is known to have concerns about the safety of undersea cables linking the UK to the rest of the world, and wrote a report on the threat in 2017 when he was a backbench MP. In addition to the task force, the defence secretary says he is sending another destroyer, HMS Diamond, to the middle east to join the five Royal Navy vessels already there.
HMS Diamond will be protecting trade routes and merchant vessels in the Gulf which have been increasingly targeted by Houthi rebels from Yemen since the Hamas attack on Israel on October 7.
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