Image caption Great whites can migrate distances of thousands of kilometres each year

Great white sharks could be "occasional vagrant visitors" to waters around the British Isles, according to an expert.

Richard Peirce, chairman of the Shark Trust, said the conditions and availability of prey made British waters an ideal hunting ground for the feared predator.

Mr Peirce said: "The real surprise is that we don't have an established white shark population, because the conditions here mirror those in parts of South Africa, Australia and northern California.

"Research has shown that white sharks tolerate water temperatures in a range which would make British waters perfectly suitable for this species."

British waters are home to many species of predatory sharks including blue and mako sharks which have been spotted off southwest England in the summer and threshers and porbeagles which are year-round residents.

There have also been sightings of other sharks in British waters over the summer. Earlier this month fisherman Jim Millar spotted a 15ft (4.5m) thresher shark off Dartmouth in Devon, where they are very rarely seen.

Another fisherman caught a 300lb (21 stone) porbeagle shark off the coast of Donegal, Republic of Ireland, last month.

And there were two separate sightings of what was believed to have been an oceanic whitetip shark, a species also known to attack humans, in St Ives, Cornwall, in June, although very few shark experts believe the sightings were oceanic whitetips.

Mr Peirce believes it is only a matter of time before proof is found that the species at the top of the marine food chain, the great white shark - Carcharodon carcharias - is occasionally present in British waters.

"Great whites are highly nomadic in movement around the north Atlantic so it's reasonable to say there's a good chance they may stray into British waters.

"I do suspect we do get the occasional vagrant visitor."

Mr Peirce claims he almost proved there is a great white occurrence in the UK with a photograph of a shark caught off the north east coast of Scotland.

"I sent the photo to some of the world's leading experts but as soon as they heard it was caught off Scotland they started looking at what else it could be."

Mr Peirce has investigated more than 80 reported sightings of great whites in British waters over the past 14 years but only seven were found to be credible.

'Compelling evidence'

A fisherman in Cornwall reported a great white sticking its head out of the water - known as "spy-hopping" - in the 1970s and fishermen onboard three different boats, also off Cornwall, described a sighting of a great white within three weeks of one another in 1999.

Mr Peirce said: "The reason the evidence is so compelling is that it's from independent witnesses who do not know each other on different boats.

"The problem is these things happen in a flash. Unless the shark jumps right out of the water or is caught, all we'll see is a dorsal fin sticking out the water.

"The closest capture of a great white was off La Rochelle (in western France) about 200 nautical miles from UK shores which is no distance to them."

Image caption In 5,000 hours of observing Seawatch SW has only seen one predatory shark

However Dr Russell Wynn, co-ordinator of the SeaWatch SW project and a senior marine scientist at the National Oceanography Centre in Southampton, said the odds of a great white being found in British waters were extremely low as the creatures are very rare in the northeast Atlantic.

The SeaWatch SW survey team has spent more than 5,000 hours scanning the seas off southwest England in the past five years but the only predatory sharks seen have been single blues and threshers.

"The only large shark the public are likely to see is the harmless plankton-feeding basking shark, which can grow to over 10m long and is occasionally seen leaping out of the water," he said.

But Dr Wynn accepted there was a small chance of a great white sighting off the British coast.

"It's certainly not impossible that a great white could be seen or caught in British waters one day, as we know they occur off southwest Europe in very low numbers."

But despite the recent sightings, the opportunity of seeing sharks is decreasing year on year.

Research carried out by Dalhousie University, Nova Scotia, Canada, in the Western Atlantic indicate serious depletions of more than 50% for many shark species.

Mr Peirce said: "Unless we do something about shark mortality in the Atlantic we won't be having this conversation in 50 years time."