Sailing off the south coast of England, the Russian trawler known as the Yantar carried its usual array of hi-tech equipment.

In photographs released by the Ministry of Defence, a large radar dome can be seen behind two masts bristling with antennae.

Officially, these allow the 108 metre-long craft to monitor ocean currents, befitting a vessel the Kremlin maintains is part of its oceanographic research fleet.

But it was the ship’s more nefarious purposes that prompted a rare display of British naval power on Jan 20, when the Yantar was confronted by a British warship, HMS Somerset, and patrol vessel HMS Tyne.

Humdrum though it may appear, the Yantar is known to carry two submersibles that can dive down up to 6,000 metres, allowing their crew to map, monitor and potentially sever the undersea cables that transmit data around the world.