As many as 20 dolphins have died and 80 more are in danger in the greatest stranding of marine life on Britain's coast for nearly 30 years.

Rescue teams described the scene near the Cornish coast this morning as “carnage with bodies everywhere” and warned that more could die.

The distress calls of the pod of dolphins, which swam upstream into a river on the south coast of Cornwall this morning, lured many others to swim in behind them, prompting a major rescue operation.

Coastguards said the first pod swam up the Percuil River near Falmouth, Cornwall, before it was beached in Porth Creek, it is believed that almost 20 of them have died.

Liz Sandeman, director of operations at Marine Connection, a charity for the protection of dolphins, said that at least 80 dolphins had been drawn into Falmouth Bay away from their deep ocean habitat.

"These animals should not be in shore, to get 100 I would say there is a problem, " she told Times Online, "Once these animals are out of the water they are too heavy and it puts huge pressure on their organs. They would basically be killing themselves by squashing."

Post-mortem examinations would determine why the animals had come into shore and died, but a problem further out to sea could be to blame, she said.

Sarah Dolman, science officer at the Whale and Dolphin Conservation Society, said that most of the dolphins were the Common breed, which are more regularly seen off the British coast rather than the more unusual Striped variety as first thought.

But she said it was "unusual" to see them in such numbers, "it looks like they were chasing fish and got caught out by the tide," she said.

Tony Woodley of the British Divers Marine Life Rescue (BDMLR) said the recent hot weather made the rescue effort a race against time. "Out of the water in the heat their skin crisps and starts cracking and they will dehydrate very quickly, it's absolutely baking down there and that's compounding the issue," he said.

A spokesman for the Falmouth Coastguard said “There are now many more following the same route in. We are not sure why they are all swimming up the river – it seems they are following the distress calls of the first pod.”

Teams of conservationists, divers, coastguards and local government officials are now in the area attempting to send the dolphins back out to sea.

Three volunteer members of the Falmouth RNLI Atlantic 75 lifeboat helped five dolphins back to deeper water this morning after two of them got in to the water alongside the dolphins.

They said many of the dolphins were already dead, with others struggling in the shallow waters or on the river edge.

Trevor Weeks, national co-ordinator of the BDMLR, said it was a "unique event" and the dolphins had most likely been attracted by swarms of fish feeding on algal bloom burgening after a weekend of warm weather.

But he added that "pollution and sonar activities by the navy" could have caused problems for the dolphins, pushing them in to shore and causing them to get lost. "In an estuary like Falmouth with so many creeks and turns it can get confusing and difficult for them to use their echolocation to navigate."

The Whale and Dolphin Protection Society say the dolphins may have come ashore because of disease or being caught up in fishing nets.

Dave Nicollm, helmsman of Atlantic 75 said: “It’s a horrible scene of carnage with bodies everywhere, but we are doing our best to help and will continue to support the expert groups. We have been trying to help those who are alive and have already succeeded in getting five back into the water. We think the pod have been attracted by the cries for help from those that are stuck in the creek.”

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