An unusually high number of dead basking sharks washed up on Cornish beaches have left experts mystified as to what prompted the sudden deaths.

The bodies of six basking sharks were found around the Cornish coastline in the space of three weeks - the highest frequency of deaths ever recorded by the Marine Conservation Society (MCS) shark watch group.

Five were found in the space of a week, with the latest discovered just two days ago at Perranporth, North Cornwall. Others have been washed up dead, at Gerran Bay, Coverack, Roseland Bay and the Fal estuary.

The MCS, which began shark watching 17 years ago, said the number of deaths was "highly unusual and very worrying".

The society appealed to coast walkers and sea users to look out for further stranded sharks - alive or dead.

"The sooner we get information, the sooner we can get someone down there to have a look and establish the cause of death," said MCS biodiversity projects officer Joana Doyle.

Since the MCS basking Shark Watch Project began in 1987 - when the public were first asked to report sightings of sharks - there have been only 61 reported deaths. The project had never encountered so many deaths in a single week.

Basking sharks are protected sea mammals which can grow up to 11 metres long and are the biggest fish to be found in UK waters.

The latest body found two days ago at Perranporth was 6.1 metres long - the biggest of all the sharks washed up dead.

Ms Doyle said the cause of death had not yet been established.

There were marks on a fin which implied it had been caught in a net, but it could not be deter mined whether this was before or after death.

One of the creatures was thought to have possibly drowned after becoming entangled in mooring ropes, but the deaths of the others remained a mystery.

Collisions with vessels were a possibility as the sharks cruise slowly when eating.

The worrying death toll comes after a widespread concern over the number of dolphin deaths on the Westcountry coastline this year. More than 250 bodies of dolphins have been found since January. Their deaths are blamed on pair teams trawling for bass during the winter months.

David Ball, of Cornwall's Silver Dolphin Marine Conservation and Dive Centre, records all instances of dolphins found dead on beaches, which are then collated by the Cornwall Wildlife Trust.

He said: "People have sent us photos and information about the basking shark deaths and we would want them to continue to do so. We need to know as much as we can about these deaths so the MCS can put together an idea of how they are happening.

"People appear to be concerned about basking sharks because of the high number of dolphin deaths and they worry that the two could be linked. There is no evidence of this so far. They could be dying of pollution.

"When you think what goes on out at sea with ships emptying and these sharks eating the plankton from the waters, it's important to know what is happening."

Mr Ball said it was just as vital for the public to contact them about sightings of live sharks.

"If people spot schools of basking sharks we would like to hear about it," he said. "If fishermen find them tangled in their nets they can give us a shout and we can come out and release them."

The seas around Devon and Cornwall are a hot spot for basking sharks - the second biggest fish in the world. In 1998 a school of 500 of the creatures, which can weigh up to seven tonnes, was spotted off The Lizard peninsula in Cornwall. In May this year there were 116 sightings of the harmless, plankton-eating sharks off Cornwall.