A Great White shark is believed to have been spotted by a holidaymaker off the Cornish coast.
Expert Richard Peirce says the shark cannot be conclusively identified
Richard Fletcher, from South Yorkshire, filmed a shark breaching in the water near a pod of dolphins about 200 yards (182m) from Porthmeor Beach, St Ives.
Cornish shark conservationist Richard Peirce, who saw the footage, said the possibility it was a Great White could not be ruled out.
But coastguards have dismissed the claims as "scare-mongering".
The footage shows an 8ft-long shark breaching almost three-quarters in the water in the vicinity of a pod of dolphins.
When Mr Fletcher returned home to Rotherham he spotted the shark among the dolphins he was videoing.
Mr Peirce, chairman of the Shark Trust, said it was impossible to make a conclusive identification and that it could have also been either a Mako or a Porbeagle shark.
He said: "If one accepts that White sharks are occasional vagrant visitors in UK waters, these animals could not be ruled out."
A spokesman for Falmouth Coastguard said there had never been a confirmed sighting of a Great White shark off Cornwall.
"We get Basking sharks and the odd sighting of a Porbeagle shark which looks similar fin wise to a Great White but they are a lot smaller and completely different.
"The poor tourist industry this year is having a really hard time. The last thing we need is scare-mongering over some footage."
British waters contain 33 different species of shark, of which Blue and Mako are considered dangerous to man, according to Mr Peirce.
A group of fishermen claim to have seen a Great White in 1999 although it has never been proven.
A 14-year-old amateur marine biologist also claimed she saw a Great White off the north Devon coast in 2003.