A snorkeller off the coast of southern England has suffered a suspected shark bite, coastguards said on Tuesday, in what appears to be an extremely rare incident.

The Maritime and Coastguard Agency said it sent a rescue team to Penzance in Cornwall "to meet a snorkeller who suffered a suspected shark bite" after the accident was reported last Thursday.

"It is believed the swimmer suffered a leg injury," the coastguard service said, adding that the snorkeller had been treated by the ambulance service.

A Cornish tour agency that arranges snorkel trips to watch blue sharks confirmed a participant had received first aid after an "incident". 

Blue sharks which grow to nearly four metres long are summer visitors to UK waters. They are hunters, which migrate through oceans, eating fish and shellfish.

The blue shark is classed as "near threatened" by the International Union for Conservation of Nature because it is hunted for its flesh and fins and also gets caught accidentally in fishing nets. 

Blue Shark Snorkel Trips, based in Penzance, wrote on Facebook, "We have had an incident."

"First aid (was) carried out on the person involved."

"Following advice and assessment from the coastguard, the person walked off the boat and received further treatment ashore," it added.

The person injured was not named but the company quoted them as saying it was a "very scary incident" but added, "I don't for a second want this freak event to tarnish the reputation of an already persecuted species."

"These occurrences are extremely rare", the company said.

It takes snorkellers to swim with blue sharks 10 to 20 miles offshore, according to its website.

It warns that blue sharks are "apex predators" with no natural predators and participants "swim with them knowing there is a risk".