A Russian submarine has travelled through the North Sea on its way to the Mediterranean.
Patrol ship HMS Mersey has been tasked with keeping watch on the surfaced Kilo-class diesel powered attack submarine RFS Rostov Na Donu during its journey from the Baltic.
A navy spokesman said: "Mersey reported on the movements of the Russian Black Sea Fleet submarine, so that Nato allies could track her progress as she continued her onward journey."
Commanding officer of HMS Mersey, Lieut Cdr Will Edwards-Bannon, said: "All of us in Mersey are very proud of the part we play, here in our home waters, in fulfilling the Royal Navy's role of protecting our nation's interests.
"No patrol is ever the same as the last and we have conducted a particularly varied range of missions recently, culminating with the tracking of this Russian submarine as it passes the UK."
Mersey was also on duty when the Kilo-class submarine made the reverse journey in October last year, as it returned from operations in the Mediterranean back to the Baltic.
The navy spokesman said: "This latest operation comes amid a busy time for the Royal Navy's patrol ships, with Mersey and her sister ships HMS Tyne and HMS Severn all on maritime security patrols in the seas around the UK.
"Royal Navy patrol ships are involved in a range of missions, including fishery protection, which has been their responsibility for many years, but also coordinating with the Joint Maritime Security Centre to share information relating to suspicious or dangerous activity at sea alongside the Maritime and Coastguard Agency and National Crime Agency."
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