Young leatherback turtles have been spotted off the British coast this summer, reflecting a possible rise in sea temperatures, according to conservationists.

While adult leatherbacks travelling from the Caribbean have frequently been found in British waters, sightings of juveniles, which are under a metre long, have rarely been recorded.

Conservationists say the creatures have been seen up to three times this summer off Cornwall, which provides further evidence of rising sea temperatures.

The first was reported in early August by a windsurfer at Constantine Bay, while another was spotted a week later off the popular Fistral beach in Newquay.

The Marine Conservation Society (MCS) wants people to report any sightings to help track the species, which lives in the Atlantic and comes to Britain to eat jellyfish. Peter Richardson of the MCS said it was surprising to see juveniles, which had not developed the fat reserves of their parents, off Britain.

"Current thinking suggests that unlike the adults, juvenile leatherbacks are warm-water animals that have not yet developed the fat reserves and other characteristics that give the adults the ability to survive our chilly seas. But these recent sightings suggested that this may not be the case. The presence of these small turtles may even be a symptom of our warming seas.''

The adult is typically covered in a reptilian equivalent of blubber, which helps it to maintain its body temperature. The highly timid creatures rarely rise to the surface near human beings, so sightings are rare, but they have been washed onto British shores after eating plastic bags and starving. Some are caught up in ropes or fishing nets.

Experts say it is highly unlikely the juveniles were a result of species breeding on British shores. The leatherback breeds in the sand and requires temperatures of over 22C. It will typically return to the same beach to lay eggs year after year.

It is the largest turtle and can reach three metres long and weigh up to a ton. Unlike other turtles, the leatherback has no visible shell but a "carapace" of hundreds of irregular bony plates covered with a leathery skin.

The endangered species lives off the West Indies, Florida, South America, Senegal, Natal, Madagascar, Sri Lanka and Malaysia.