The Royal Navy was carrying out live-firing exercises just hours before 26 dolphins died in the biggest mass stranding of the species in Britain, it has been claimed.

Marine experts trying to find out why the pod of dolphins tried to beach themselves on the shores of Percuil river, near Portscatho, Cornwall, say they could have been panicked by an "underwater disturbance".

The dolphins were found dead early on Monday morning.

Falmouth Coastguard said that there had been "heavy" Royal Navy activity in the Falmouth Bay area and suggested it had continued past midday on Sunday.

A spokesman said: "There have been continual live-firing exercises up until yesterday [Monday] evening when the exercise was cancelled."

It is understood that the exercises have been going on for two to three weeks and were carried out day and night.

The British Divers Marine Life Rescue (BDMLR), which co-ordinated the rescue of several dolphins, said they received reports from the public that an explosion took place at sea over the weekend.

Such an explosion could have panicked the dolphins - who were away from their natural deep water habitat - and sent them up the river to shallow waters.

"It does not mean that the Navy is to blame, but it would be naive of us to ignore the activity that has been going on," said the BDMLR.

Comment: More on the mysterious explosion from Reuters:
"Investigators are exploring several possible causes, including the theory that the dolphins had become distressed by naval sonar equipment or were frightened by an unexplained explosion several residents reported hearing late on Sunday.

Claims the animals had moved inland to feed on fish, attracted by a large algae bloom caused by a recent bout of hot weather, were dismissed after no food was found in their stomachs.

While confirming it had a "presence" in the area, the Royal Navy said no training or official activity had been conducted there since last Thursday.

A Navy spokesman also said officials had no knowledge of an explosion involving any vessels.

Despite residents contacting two separate conservation groups reporting hearing loud explosions, local police and the coastguard said they had no record of such an incident."

An MoD spokesman initially confirmed that live-firing exercises were taking place until midday on Sunday in Falmouth Bay, close to the entrance to the Percuil river. The exercises involved a submarine.

However, the MoD later issued a statement claiming that the last live firing by the Royal Navy in the area had actually taken place on Friday - 60 nautical miles from Falmouth.

"These live firings are routine training," the spokesman said.

The MoD also disclosed that a survey vessel was conducting trials using high definition sonar for sea bed mapping trials off the coast of Falmouth at the time of the dolphin incident.

However, the spokesman insisted: "It is considered extremely unlikely that this operation could have affected the mammals in any way."

Post-mortem examinations on some of the dead dolphins have yet to reveal any concrete evidence as to why the dolphins had swum up the river.

They appeared to have been well fed and there were no obvious signs of disease or poisoning. The tests are expected to take weeks or months to complete.

Other theories put forward yesterday included the possibility that the dolphins may have been trying to escape a killer whale or were seeking food.

The BDMLR chairman Alan Knight, said: "There is a possibility that these animals have been frightened by something which means they have panicked and beached themselves.

"The fact that they have beached in four separate sites is very unusual. It indicates to me that there is some kind of disturbance."

David Jarvis, also from the charity, said: "It could have been something to do with sonar or it could have been that there was a killer whale out there that frightened them."

Sarah Dolman, of the Whale and Dolphin Conservation Society, said: "Sonar is the most plausible [reason] based on previous strandings around the world."