Conservation experts say the discovery of a leatherback turtle in the North Sea is exciting and significant.
Leatherback turtles were only seen 23 times in UK waters in 2003
Eyewitnesses have reported seeing a six-foot creature swimming 100 metres off the Norfolk/Suffolk coast.
Peter Richardson, from the Marine Conservation Society, said the species was the largest turtle in the world.
"It travels to British waters in summer to feed on jelly fish but is normally seen on the west side near the Atlantic so is an exciting find this far east."
Some experts believe that the more frequent sighting of the leatherback turtle is the result of global warming.
Mr Richardson said this may be the case and that evidence was being collected.
"It is rarely seen in the colder North Sea away from the Gulf Stream.
"We do not yet have enough information to decide if warmer sea temperatures are a contributory factor but we believe these have had an influence.
"We need more records to see where the trend goes next."
Peter May, a teacher from Brighton, saw the creature off Corton near Lowestoft.
He said: "At first I thought it was a body because it was the size of an adult human.
"Its head was sticking out of the water and I saw the distinctive ridge on its back.
"It was not close enough for a picture but it dived and disappeared."