Scientists fear the UK's only known resident population of killer whales is on the verge of extinction.

Experts have warned that only nine older animals are still alive in an Orca group seen off the north and west of Scotland round to the west of Ireland, and there is no chance of them producing new calves.

The Hebridean Whale and Dolphin Trust said no live calves have been recorded since research began almost two decades ago and warns that the group's female killer whales are no longer able to reproduce.

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It blames this on their social isolation which makes them unlikely to mate with any killer whales from other populations, whereas other pods can be up to 40-strong.

A spokeswoman for the Mull-based charity said: "This means the conservation status of this group is critical. As sad as this is, the loss of this group has severe consequences, resulting in the loss of an evolutionary significant group of individuals.

"Through our research, we have discovered that this group is physically different to other killer whale populations in the north-east Atlantic, suggesting separate ancestry. They are bigger in size by about a metre and have different tooth wear."

Research has established that of the nine Orca, four are male and five female. Genetic analysis indicates that this group is more closely related to a pod of Antarctic killer whales than those found in Atlantic waters.

The trust spokeswoman said that a comparison of the teeth in these killer whales with those in other populations suggested this unique population was feeding exclusively on other cetaceans, such as harbour porpoises and minke whales.

National Geographic magazine describes killer whales as the largest of the dolphins and one of the world's most powerful predators. They feed on marine mammals such as seals, sea lions and even other whales, making use of teeth that can be four inches long. Orcas are found from the polar regions to the Equator.

The trust is encouraging the public to report their sightings of marine mammals via its website "to contribute to our growing understanding of these magnificent creatures". It says: "There is still much to learn about the West Coast Community. Only with continued study can we better understand the only resident population found in British waters."

Meanwhile, the trust's biodiversity officer, Olivia Harries, said the studies suggested the killer whales had become isolated from other groups in the north-east Atlantic.

She added: "The group demographics are highly skewed to older individuals and there is an unusually high ratio of adult males.

"Recruitment is therefore unlikely and the females are probably post-reproductive.

"This means that these killer whales are due to go extinct in our lifetime."