The dolphins that died after becoming beached in Cornwall had ingested debris and mud, leading one of the scientists who examined some of the corpses to compare their deaths to a "mass suicide".

Vic Simpson, who has been involved in the postmortem examinations of some of the 26 dolphins that died, said yesterday: "On the face of it, it looks like some sort of mass suicide - but the question is why?

"The dolphins had swallowed and inhaled big chunks of mud from the estuary. Their lungs and stomachs were full of it. That is very bizarre indeed."

Simpson, who founded the Wildlife Veterinary Investigation Centre in Truro and is examining the dolphins on behalf of the Zoological Society of London, added: "We have seen strandings on beaches, sometimes with five to seven dolphins, but never on a scale like this."

Dozens of common dolphins were found at four sites at the river Percuil in south Cornwall on Monday. Rescuers were able to usher some back out to sea, but 26 could not be saved.

There are several theories, one of which is that the dolphins may have been upset by some sort of underwater disturbance.

The Royal Navy was operating nearby at the time of the beachings. There had been live firing and a submarine had been in the area. The Ministry of Defence also said one of its vessels had been using short-range sonar to scan the seabed. But a spokesman said the trial was 12 miles off Falmouth and added: "It is considered extremely unlikely that this operation could have affected the mammals in any way.

"We take our environmental responsibilities very seriously and always carry out an environmental impact assessment before putting energy in the water."

Other theories suggest the dolphins may have been frightened by a killer whale or that they had suffered some sort of infection. It is not thought they became trapped after chasing fish into shallow water, because the stomachs examined were not full of fish.

The idea of dolphins committing suicide is a controversial one. In Iran last year 152 dolphins washed up on the coast, leading people there to claim they killed themselves. But most scientists believe this is an example of anthropomorphism.

Liz Evans-Jones, a zoologist at the Natural History Museum, said: "Whales and dolphins strand themselves for a number of reasons and we're not sure yet what happened with these dolphins.

"There has not been a mass stranding in Cornwall since November 2001, when 16 common dolphins were spotted floating dead out to sea just off Looe."