Whales beached on British shores will be humanely killed as soon as possible after they are found, under new official guidelines. Scientists and conservationists say recent research shows that trying to return stranded whales to their ocean habitats was impractical in the UK and that any attempts to do so merely prolonged the suffering of the animals.

The Marine Animal Rescue Coalition, which includes the RSPCA and is responsible for handling stranded marine animals, worked with scientists from the Zoological Society of London (ZSL) on the new policy, announced today. It states that stranded whales should be put down by a lethal injection of an opiate unless there were clear reasons not to do so. The policy would apply to deep-sea creatures such as beaked and sperm whales but not to dolphins or porpoises, which are also sometimes found stranded on UK shores.

"We now have a clear direction for those who respond to the strandings, " said Tony Woodley of the RSPCA. "The presumption will be euthanasia unless the animal can be refloated in a very short period of time into oceanic-depth water, which is extremely unlikely in the British Isles because of the logistics of getting animals to those waters in that time period."

Paul Jepson of ZSL, who has led much of the research on stranded marine animals in recent years, said: "Between 2002 and 2006, there were 30 sperm whales and 24 beaked whales reported stranded in the UK and none of them survived."

Marine animals can become stranded for several reasons, including the effects of disease or pollution or as a result of trauma from collisions with boats or other animals. Once onshore, their bodies undergo rapid deterioration.

Recent research on blood samples taken from stranded whales, including the famous beaked whale that swam into the Thames in January 2006, all point to the same causes of death: dehydration and irreparable muscle and kidney damage.

Deep-sea marine animals such as beak or sperm whales use an oxygen-carrying protein called myoglobin in their muscles to carry much of the oxygen they need to swim deep in the sea. When they are stranded on land, their dense muscles become compressed under their own weight. The cells die and release the myoglobin into the bloodstream, which is toxic for the kidneys.

Stranded whales were also found to be dehydrated because their only source of water is the food they eat. When the whales get lost in the North Sea or other relatively shallow waters, they cannot find anything to eat. "They become so weak with dehydration, they strand and a whole cascade of physiological compromises begins," said Jepson.

Because of this rapid deterioration, which can happen within hours and become irreversible well before the stranded whales could be moved back into deep waters, scientists have concluded that it is virtually impossible to save the creatures. Dolphins and porpoises deteriorate more slowly, and live in much shallower waters, so refloating them is often easy.

"We like to base good policy on science and this is one example where the science shows us that the animals, when they strand, are severely compromised," said Adam Grogan of the RSPCA.

He said the lack of recent strandings made it a good time to announce the change in policy because emotions tended to run high when the whales were in trouble. "It's only fair to tell everybody that in the future it's very likely that, to put it out of its suffering, euthanasia is likely to be the most likely option."