Map of confirmed sightings of Chinese mitten crab in Norfolk

Chinese mitten crabs are a non-native species with distinctive claws that live in rivers, canals and estuaries and pose a serious threat to the ecosystem.

They are typically grey-green or brown in colour and have dense "fur" around white-tipped claws. Adults can grow as large as a dinner plate.

Adult crabs can grow as large as a 30cm dinner plate (Image: Newsquest)

With more than 800 recorded sightings in the UK so far, their spread has been confirmed in Norfolk's rivers and waterways - prompting concerns for native wildlife. 

The crabs damage riverbanks, compete for food with native species, block vital water outlets and damage fishing equipment with their claws. 

The National History Museum has since established its "Mitten Crab Watch" and used a live map to track the species' invasion. 

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So far, mitten crabs have been spotted in large numbers around west Norfolk, particularly in Downham Market, King's Lynn and Hunstanton. 

There have been five additional sightings between Wells and Sheringham, and three more in Great Yarmouth, Reepham and Oby.

The Norfolk Non-Native Species Initiative (NNNSI) is trying to track their spread across Norfolk and would like the public help if they think they may have seen any.

Sightings of Chinese mitten crabs have been confirmed in east, north and west Norfolk (Image: NBN Atlas)

Liam Smith, nature recovery officer at the NNNSI, previously said: "The news about Mitten Crabs at Buckenham and Oby is worrying.

"The species is already known around the north west coast of Norfolk but currently hasn't properly colonised the Broads system.

"If that were to happen it could be really bad news."

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Due to their preferred habitats, anglers are most likely to see or catch the crabs and are encouraged to notify the relevant authorities immediately.

A Defra spokesman said that there is no legal fishery for the species, meaning if a Chinese mitten crab is caught it cannot be sold live for human consumption.

People are advised that if they see a Chinese Mitten Crab in the wild, take a picture of it and report its location to the Norfolk Non-native Species Initiative, the Marine Biological Association or the Non-native Species Secretariat as soon as possible.