One of the world's rarest turtles has hatched at a British zoo and is currently the size of a matchbox.

The tiny six-week-old Vietnamese box turtle, a critically endangered species, weighs just half an ounce.

It hatched after being kept at a constant temperature in an incubator at Bristol Zoo Gardens for 85 days.

The turtle is so precious that it is being kept off-show from zoo visitors in a climate-controlled quarantine room.

Once old enough, it will join the six adult box turtles in the custom-built Asian turtle breeding room in the zoo's reptile house.

Tim Skelton, curator of reptiles at the zoo, has cared for turtles for more than 40 years.

"This is a very difficult species to breed so I am thrilled with the arrival of this baby. It comes after a lot of hard work," Mr Skelton said.

"Little is known about this species so we can learn an awful lot from this baby to improve our chances of breeding more in the future.

"These are secretive animals so we are keeping it in a warm, humid and quiet room with a constant temperature, in an enclosure to replicate its natural habitat where it can burrow among the soil and leaves."

The turtle is the second bred by the zoo, which has kept the species for 12 years. The zoo's first Vietnamese box turtle hatched in 2012 and is thriving on a diet of snails, worms and chopped fruit.

Bristol Zoo is believed to be the second zoo in Europe to have bred the species.

An adult box turtle weighs around two pounds, measures eight inches long and can live up to 50 years old.

Box turtles are mainly terrestrial, although they will enter shallow water to hunt and soak.

They are hunted for their meat, for use in traditional medicines or as pets and are listed as critically endangered on the International Union for Endangered Species red list.

Bristol Zoo, a conservation and education charity, is working with the Turtle Conservation Centre in Cuc Phuong national park in Vietnam to help safeguard the species.