The Cleveland Metroparks Zoo in the United States, which sponsored the research, made the announcement in a statement.

"This is an incredibly important discovery because the Swinhoe's turtle is one of the most critically endangered species of turtle in the world," Doug Hendrie, the Vietnam-based coordinator of the US zoo's Asian Turtle Program, said in the statement.

"This species has legendary status among the people of Vietnam, so this is perhaps an opportunity for the legend to live on."

Researcher Tim McCormack of Cleveland Metroparks Zoo declined to provide the name of the lake or give other details of the turtle's location for fear it would be hunted and sold into the wildlife trade.

The zoo said that only three other specimens of the turtle are known to scientists, two at zoos in China and one in the storied Hoan Kiem Lake (The Lake of the Returned Sword) in the centre of Hanoi.

The turtles can weigh up to 136 kg (300 pounds), measure up to 0.9144 metres (3- feet) and live more than 100 years.

But researchers say they have become virtually extinct because of hunters who killed them for food, loss of nesting habitats and pollution. The Asian Turtle Conservation Network says as many as 15 million turtles are traded a year in Asia, most of them ending up in China.

The reptile in the Hanoi city lake has a special place in Vietnamese folklore and whose appearance some believe to be a portent of an extraordinary event.

The legend tells how the 15th century Emperor Le Loi used a magic, divine sword to drive out Ming invaders from China.

A giant turtle emerged while Le Loi was boating on the lake and told him to return the sword to the Dragon King. The weapon shot from its sheath into the mouth of the turtle, which disappeared underwater.

Since then, the lake previously called Ho Luc Thuy or Green Water Lake became known as The Lake of the Returned Sword.

More than seven years ago, Vietnamese zoologist Ha Dinh Duc named the reptile, estimated to weigh about 200 kg (440 lb), Rafetus Leloi, in honour of the emperor.