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Teams were called to a Kent beauty spot after a porpoise washed up onto a beach.
The creature's body was spotted yesterday at Walpole Bay in Margate by shocked passers-by.
Experts from British Divers Marine Life Rescue were sent to the scene, where they identified the animal as a juvenile female.
"There were no obvious injuries pointing to a cause of death," a spokesman for the charity said.
"But the animal does appear to be in poor nutritional body condition."
The body will be collected by staff from the Natural History Museum, before a post-mortem is carried out to establish the cause of death.
The mammals, which are often mistaken for dolphins, face threats from water pollution, noise from shipping vessels and fishing nets, which they can become entangled in.
"Harbour porpoises are fairly common around the Kent coast," the spokesman added.
"We do see occasional strandings throughout the year."
This comes after a harbour porpoise was spotted swimming in the Thames.
It was seen frolicking in waters near Gravesend Town Pier.
Following the discovery in Thanet, district council officials are warning residents not to touch any animals that have washed up from the sea.
"Please do not attempt to move any stranded dead mammals yourself," an authority spokesman said.
"Keep dogs and children away from them also as they may have infectious or transferable diseases."
Harbour porpoises can grow up to two metres long, and can be identified by their dark grey upper side and white belly, according to British Divers Marine Life Rescue.
They also have a blunt head with no beak-like features and small fins.
Porpoises are usually found in groups and feed mainly on fish, such as herring, sprats and whiting.
British Divers Marine Life Rescue is urging anyone who spots a stranded seal, dolphin, porpoise or whale to contact its hotline on 01825 765546.
Discoveries of stranded dead mammals can also be reported to the council by calling 01843 577000 or emailing streetcleansingteam@thanet.gov.uk.