NAVY warships are to be equipped with a £2.5m scanning system to spot marine mammals after post-mortem tests linked the death of beached whales to military sonar.

Advanced software will be used to adapt the ships’ sensors to “listen” for the sounds of more than 100 species, including whales, porpoises and dolphins. If any are detected, the sonar will be switched off.

The development comes amid evidence that navy sonar may be responsible for the death of whales and dolphins, confusing them into surfacing so quickly that they suffer fatal attacks of the “bends”.

Scientists in Britain and the Canary Islands have established that the deadly condition, which afflicts human divers when they ascend too fast, can also strike marine mammals.

Sonar has been identified as the likely cause of death in four beaked whales stranded on the southern coast of Spain in January, according to preliminary results of post-mortem tests released last week.

This weekend the Royal Navy admitted using mid-range sonar on a training exercise near Gibraltar two days before the whales became stranded on the Almeira coast.

Hundreds of whale strandings have now been linked to sonar, while the United Nations has warned it is a big threat to marine mammals. In Britain autopsies on 13 stranded whales, dolphins and porpoises since 1990 have revealed signs of decompression sickness.

The use of military sonar appears to interfere with the echo-location system the animals use to navigate, leaving them so disoriented they misjudge depths and swim to the surface too quickly. The result is decompression sickness, in which nitrogen dissolved in the blood expands to form bubbles that fatally damage blood vessels and internal organs.

Since January four sperm whales have become stranded on Britain’s North Sea coast, although their bodies decomposed too quickly for tests to establish a cause of death However, Antonio Fernandez, a veterinary pathologist who examined the whales stranded off Spain, said: “We believe acoustic activity caused by sonar creates panic among the whales and they go very fast up to the surface.”

Fernandez, of the University of Las Palmas in Gran Canaria, added: “They suffer from lesions consistent with acute trauma due to nitrogen bubble formation resulting from rapid decompression.”

The new monitoring system, devised as part of a £2.5m project by the navy to protect marine mammals, makes use of the ships’ “passive sonar”, which is normally deployed to listen for enemy vessels.

The Ministry of Defence (MoD), working with the Sea Mammal Research Unit at St Andrews University, has created a “sonic database” of calls used by 120 marine species as part of the scheme. It will be introduced in 2008.

Sounds picked up within a two-mile radius by the passive sonar on warships will be checked against the database. If any calls are heard 30 minutes before active sonar is scheduled, it will be called off.

Active sonar emits a pulse of sound that bounces back when it meets an object, revealing the location of vessels.

Matt Summers, a marine environment expert from the MoD’s defence, science and technology laboratory, said: “The navy is committed to being a good environmental steward.”

The navy says sonar use has declined in recent years because of cutbacks in the fleet. Increased shipping and North Sea oil exploration are also blamed for disrupting whales.

But the move has not allayed the concerns of environmental groups, who warn that a powerful system being introduced poses a significant threat. Sonar 2087, which has cost £160m to develop, has already been installed on two frigates, with plans to fit it on four more this year.

The low-frequency system will operate at long range and the MoD admits it has the potential to be harmful to marine life.

Liz Sandeman, co-founder of Marine Connection, a conservation group, said: “Low frequency sonar can travel for hundreds of miles, yet the marine mammal detection system will only work for two miles.”

The plight of whales became a big issue in January when a northern bottlenose died after becoming disoriented and swimming up the Thames. Natural History Museum data show the number of whale, dolphin and porpoise strandings in Britain has doubled from 360 in 1994 to 782 in 2004.

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