The children were on a sea safari in Torquay when their teacher spotted the predator and alerted boat captain Ron Hughes.

However, Mr Hughes suspected something was not quite right when he steered the boat towards the two-feet tall object and noticed it had not moved.

Those suspicions were confirmed when he pulled the fin out of the water to find it was attached to a wooden frame.

The fin itself had been carved from hardened insulation foam, which had been painted dark grey before being fixed to the frame and placed about 300 metres off the coast.

Writing on Facebook, Mr Hughes said: "Hmm, I wasn't fooled for long.

"Lack of movement and no tail fin made me investigate further. Looked like Great White, fin too broad for sunfish? What is it? Well someone has gone to great lengths to create a hoax Great White Shark fin.

"I can't say I'm very impressed. Pranks like this can have a very real effect on summer coastal business. Not to mention that it is basically deliberate marine littering.

"Whatever the reasons behind this stunt rest assured there has never been any evidence of Great Whites in South Devon."

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The fake shark fin was in fact an elaborate prank, with an anonymous poster on Facebook writing: "I've had fun and got the community talking.

"I just want to reassure everyone that you have nothing to fear in the sea. The recent shark was supposed to be for a personal video I didn't expect anyone to notice."

The prank was similar to the one pulled in the classic Spielberg film Jaws, where two boys strapped shark fins on to their backs to scare swimmers in Amity Island.

Eliciting one of the film's iconic lines, one of the comments underneath Mr Hughes' Facebook post read simply: "You're gonna need a bigger boat!"