A Royal Navy submarine and survey ship were carrying out training exercises close to where 26 dolphins died after apparently panicking at an underwater disturbance, the Ministry of Defence has said.
The dolphins were found beached around the Percuil river near Falmouth, Cornwall, on Monday morning.
Marine experts said it was one of the worst cases of mass stranding in the UK and added the dolphins were likely to have been panicked by "an underwater disturbance".
The MoD said there was a submarine in the area but would not comment on the training exercise. But the underwater vessel would have relied on passive sonar during the exercise and not low frequency active sonar.
The Maritime and Coastguard Agency (MCA) said it was issuing warnings to shipping about "extensive submarine activity" from Sunday morning to midday Monday.
An MoD spokesman also said it had a survey vessel using "short-range side-scan sonar" for sea bed mapping trials but added this was "extremely unlikely" to have affected the dolphins.
The spokesman also confirmed it had carried out live firing exercises off Eddystone Light, south of Plymouth on Friday.
Post-mortem tests on the dolphins are continuing and so far preliminary findings have failed to show an obvious reason why they died or became stranded.
The tests revealed the animals were well fed and there were no obvious signs of disease or poisoning.
British Divers Marine Life Rescue spokesman Alan Knight said: "We can find no conclusive evidence as to why the stranding took place or why they stranded in such numbers at different locations. My own personal conclusion is that there was some sort of disturbance that has caused the animals to panic