Banned chemical pollutants could wipe out killer whale populations within decades, a major new study has found.

PCBs (polychlorinated biphenyls) were banned 40 years ago, but experts said they were still posing a deadly threat to orcas in the wild.

ZSL (Zoological Society of London) conservationists said the pollutants threatened to "wipe out" the species in a "killer whale apocalypse".

Overfishing and man-made noise have also contributed to declining numbers.

Toxic chemical pollutants could trigger a "killer whale apocalypse" with the species likely to disappear entirely from parts of the world's oceans within just a few decades, conservationists have warned.

More than 40 years after PCBs – or polychlorinated biphenyls – were banned, experts said they were still posing a deadly threat to orcas in the wild.

And the seas around the UK are some of the most polluted in the world, researchers from Zoological Society of London (ZSL) and Aarhus University found.

They recorded rapidly declining populations of killer whales in 10 of the 19 areas investigated, with the species expected to disappear entirely from several areas within a few decades.

An Orca is examined after it washed up at Whatipu Beach on March 14, 2017 in Auckland, New Zealand. A team from the Coastal - Marine Research Group at Massey University conduct a necropsy to establish cause of death, and take biological sample to assess diet and pollutant loads in the adult male whale. Fiona Goodall / Getty Images

ZSL conservationists said the pollutants threatened to "wipe out" the species in a "killer whale apocalypse".

In the UK, numbers of orcas and dolphins have plummeted, with the killer whale population off the coast of Scotland down to just eight.

Dr Paul Jepson, co-author of the study from ZSL's Institute of Zoology, said the PCB problem in Europe was "about as bad as it could possibly get".

"All we have done is banned them and hoped they went away," he told The Independent. "The US produced even more PCBs but they are spending a lot of money on a clean-up and it is working."

Asked if this was the beginning of the end for world's orca population, Dr Jepson replied: "No, it is worse than that. The beginning of the end for killer whales began in the 1960s when high levels of PCBs were being used."

Young children get a close-up view of an Orca killer whale during a visit to the animal theme park SeaWorld in San Diego, California Thomson Reuters

The killer whale is one of the most widespread mammals on Earth and is found in all of the world's oceans, from pole to pole. But today, only the populations living in the least polluted areas possess a large number of individuals.

The seas around the UK and the Strait of Gibraltar, as well as off the coast of Brazil and in the northeast Pacific, are among the worst affected, where models show the populations have virtually been halved during the half century since PCBs have been present.