The civilian ship the MV Gary Chouest – linked to the United States Government through secretive undersea cabling and underwater listening devices – is on a port visit to HMNB Devonport.
The vessel was recently spotted off the Irish Coast and then tracked moving into the deep Atlantic waters.
Defence experts believe the vessel might be working on maintaining the Sound Surveillance System (SOSUS), which listens for passing submarines on the seabed and requires regular maintenance.
