Post-mortems on 11 of the dead creatures, who perished in shallow water after swimming up the Percuil River near Falmouth, Cornwall, have shed no light on why the 66-strong pod of dolphins entered the river.

But experts believe they may have died trying to escape a man-made underwater noise such as that caused by a navy sonar device or mining.  Forty dolphins were saved in a rescue operation.

The Navy was carrying out firing exercises involving ships and a submarine before the dolphins died, having beached themselves in panic.

Falmouth Coastguard said that there had been “heavy” Royal Navy activity in the Falmouth Bay area.

Local residents reported hearing an explosion at sea over the weekend.

The Ministry of Defence said a submarine had been on exercise in the area but it was “extremely unlikely” its operations had affected the dolphins.

The Royal Navy said no activity had been conducted since last Thursday in the area.