The collapse of turtle numbers on some of the Caribbean's most famous and glamorous islands has been so severe that in Bermuda turtles no longer breed any more.
In the Cayman Islands, the idyllic archipelago that once supported millions of turtles, biologists fear the hawksbill turtle is locally extinct.
The findings, which follow a three-year, government-funded investigation carried out by biologists at Exeter University and the Marine Conservation Society, have alarmed environmentalists.
Local fishermen and biologists have noticed the slow decline in turtle numbers for some years, but the new study has, for the first time, revealed the extent of the crisis.
Turtles were once the most important source of food and income and played a vital part in the culture of the islands. The Cayman Islands were originally settled because of their vast turtle populations, then the largest in the Atlantic.
But now, on most of the British Overseas Territories surveyed for the study, the total number of female turtles that nest on beaches each year is probably fewer than 50. Only in the Turks and Caicos Islands are turtles still plentiful, thanks to the vast sea grass beds nearby.
Gina Ebanks-Petrie, head of the Cayman Islands' Department of the Environment, remembers her grandfather making a living from turtle fishing. Now the situation is so acute, she is discussing a complete ban on all turtle harvesting - a measure already introduced on neighbouring Anguilla.
Professor Brendan Godley, who led the study, said the predicament was so severe because of the turtles' long life and breeding cycles. Most females do not begin breeding until they are 20 or 30 years old and then lay eggs only once every three years. Because of the high number of eggs lost to predators and to the elements, in some cases, only one egg in a thousand will lead to an adult, breeding turtle.
To make matters worse, an island's turtles rarely breed with another island's turtles. Females return to the beach they were born on to breed. However, many beaches, mangrove swamps and sea grass beds - all essential habitats for nesting and for feeding - are being lost to tourism developments such as marinas.
While islanders are already voluntarily cutting back on the numbers of turtles they catch and are trying to protect breeding grounds, local poverty has forced them to demand help from the UK.
Despite £250,000 having been spent on this and similar conservation projects, Professor Godley agreed the Government had to spend far more helping the territories to ensure turtle fishing could continue at a sustainable level. "Otherwise they will be lost for ever," he said.