Shark fisherman Phil Britts said he saw the fish, which he estimated was 16ft long and weighed 1,500lb, a mile off the Cornish coast at the weekend. It surfaced beside his 28ft boat Blue Fox and was seen by all seven people on board, he said.
Although Mr Britts took no photographs, experts last night did not rule out his sighting of the most feared fish of the sea. Britain's leading authority on the great white, Ian Ferguson, chairman of the Shark Trust, said that from Mr Britts' description, it was possible.
"There is no oceanographic or biological reason why great whites should not occur in UK waters," he said. He added that people should not be afraid as attacks on people by great whites were "extremely rare."
The previous closest sighting was at La Rochelle in south-western France in 1977. However, over the past decade several fish species from more southerly waters have turned up off the south-west coast of Britain, probably because of global warming, and there has been speculation that the great white might soon be among them. In January last year a large shark thought possibly to be a great white was seen off the Shetland Islands.
Mr Britts, of Padstow, said he spotted the creature as he was dangling a small shark in the water for the benefit of a photographer on board. "I looked up and 30 to 40 feet off to one side I saw this thing just rise up in the water and come right for us. I shouted and everyone saw it, and the first thing that came into my mind was it was a great white," said Mr Britts.
Mr Ferguson, who said great whites were an endangered species, appealed to fishermen to photograph it rather than hunt it.