According to newly translated instructions inscribed in ancient Babylonian on a clay tablet telling the story of the ark, the vessel that saved one virtuous man, his family and the animals from god's watery wrath was not the pointy-prowed craft of popular imagination but rather a giant circular reed raft.

The now battered tablet, aged about 3,700 years, was found somewhere in the Middle East by Leonard Simmons, a largely self-educated Londoner who indulged his passion for history while serving in the RAF from 1945 to 1948.

The relic was passed to his son Douglas, who took it to one of the few people in the world who could read it as easily as the back of a cornflakes box; he gave it to Irving Finkel, a British Museum expert, who translated its 60 lines of neat cuneiform script.

There are dozens of ancient tablets that have been found which describe the flood story but Finkel says this one is the first to describe the vessel's shape.

Fortunes were spent in the 19th century by biblical archaeology enthusiasts in hunts for evidence of Noah's flood. The Mesopotamian flood myth was incorporated into the great poetic epic Gilgamesh, and Finkel, curator of the recent British Museum exhibition on ancient Babylon, believes that it was during the Babylonian captivity that the exiled Jews learned the story, brought it home with them, and incorporated it into the Old Testament.

Despite its unique status, Simmons' tablet – which has been dated to around 1,700 BC and is only a few centuries younger than the oldest known account – was very nearly overlooked.