A 4000-year-old Mesopotamia clay tablet dealing with the ancient biblical story of Noah's ark has gone on display at the British Museum in London.

Described by the museum's tablet curator Irving Finkel as "one in a million," the sixty-line cuneiform passage on the tablet describes a circular vessel known as a coracle, not the rectangular vessel of modern mythology.

"It was really a heart-stopping moment - the discovery that the boat was to be a round boat," Finkel said at the launch of his book "The ark before Noah." "That was a real surprise."

The tablet records a Mesopotamian god's instructions for building a giant vessel - two-thirds the size of a soccer field in area - made of rope, reinforced with wooden ribs and coated in bitumen.

Etched in the clay is one of the story's key elements: It describes how the animals must enter "two by two".

George Smith, a British Museum decipherer, first identified the story known from the Book of Genesis in a seventh-century cuneiform tablet from Nineveh. The two accounts – Babylonian and biblical – were closely related. The new tablet, however, written in about 1750 BC, has startling new contents.

As Finkel describes it, "when the gods decided to wipe out mankind with a flood, the god Enki, who had a sense of humour, leaked the news to a man called Atra-hasis, the ‘Babylonian Noah,’ who was to build the Ark.

"Atra-hasis’s Ark, however was round. To my knowledge, no one has ever thought of that possibility. The new tablet also describes the materials and the measurements to build it: quantities of palm-fiber rope, wooden ribs and bath-fulls of hot bitumen to waterproof the finished vessel.

"The result was a traditional coracle, but the largest the world had ever dreamed of, with an area of 3,600 square meters and six-meter high walls. The amount of rope prescribed, stretched out in a line, would reach from London to Edinburgh!"

David Owen, professor of ancient Near Eastern studies at Cornell University, said the British Museum curator had made "an extraordinary discovery."

Elizabeth Stone, an expert on the antiquities of ancient Mesopotamia at New York's Stony Brook University, said it made sense that ancient Mesopotamians would depict their mythological ark as round.

