Construction work on the Channel Tunnel Rail Link (CTRL) in Kent has unearthed the 400,000-year-old remains of an elephant.
The skeleton was found at the site a new station
The skeleton was found on the site of the new Ebbsfleet station, an area thought to be an early Stone Age site.
Bones from other large animals, including rhinoceros, buffalo and wild horses, have also been found nearby.
The remains were preserved in muddy sediment near what was once the edge of a small lake, a spokesman said.
The elephant, which has been identified as a straight-tusked Palaeoloxodon antiquus, would have been twice the size of the largest modern African elephant.
The skeleton was also found with a number of flint tools surrounding it, indicating that it was probably slaughtered by humans.
Dr Francis Wenban-Smith of the University of Southampton, who made the discovery, said: "Only a handful of other elephant remains have been found in Britain and none of these give any indication of human exploitation.
"It is hard to imagine early humans successfully hunting a healthy specimen, but if it was already trapped in the bog, it could have been killed by early humans with wooden spears and then butchered for its meat with flint tools."
The archaeological investigation is being carried out by Oxford Archaeology, on behalf of CTRL project managers Rail Link Engineering (RLE) and its client Union Railways.
Helen Glass of RLE said: "During pre-construction investigations across the Ebbsfleet Valley we found an Anglo-Saxon mill, as well as the remains of a Roman town and villa complex.
"We thought we had found everything, but it seems the best has been saved for last."