The first orca washed up on Britain's coast in 20 years had plastic in its stomach, scientists have said, as they work to determine the cause of death.

The juvenile male killer whale, approximately 15ft long, became stranded in salt marsh in The Wash on the east coast of England.

It is the first confirmed orca stranding that the Cetacean Strandings Investigation Programme, of which ZSL is a partner, has investigated in England and Wales since 2001.

The team found a large fragment of plastic in the first stomach, though it had not killed the orca as the stomach was not blocked.

There was no evidence of recent feeding, as its stomachs were largely empty, and it is thought the animal died weeks ago.

ZSL's Rob Deaville and Matt Perkins collected blubber, liver, muscle and kidney samples from the marine mammal, which was internally mostly intact.

Teeth samples are also being investigated in order to determine the predator's age.

A spokesperson from ZSL told The Telegraph that this is a rare event "because there are so few orcas; the population has crashed."

He added: "There are two populations of killer whale, you get residents and migatory. The resident population in the UK is tiny now because the water is highly polluted.

"There are pods up in Norway and it could have been passing through the North Sea, but we cannot confirm that."

Orcas are a priority species for the scientists at ZSL because they are top predators and greatly threatened by pollution.

While the species once boasted large pods across the oceans, now only the populations living in the least polluted areas possess a large number of individuals.

Around the British Isles, the researchers estimate that the remaining population counts less than 10 killer whales due to pollution from PCBs, which are man-made chemicals used in electrical components in plastics. These are now banned in most countries and have been since the 1980s, but they take a long time to break dwon in the oceans.

As they are at the top of the food chain, killer whales whose diet includes, among other items, seals and large fish such as tuna and sharks critically accumulate PCBs.

Orcas are, however, thriving in the oceans around the Faroe Islands, Iceland, Norway, Alaska and the Antarctic, as there is less pollution.