The proof is the fanged skull of a big cat - probably a leopard - found recently in the river Fowey, near St Cleer, Cornwall, by three teenage brothers walking near the moor.
The zoo's assistant curator of mammals, Doug Richardson, said: "It is definitely genuine. There is no doubt at all." He said the skull was that of an adult big cat, which had possibly been dead for a few months.
Comparisons with other skulls would reveal whether it was a leopard or a puma, he said. DNA tests, which could take a few days, could help determine where it came from.
Mr Richardson said "credible sightings" from around the country indicated there were more wild cats at large, probably freed from private zoos.
The discovery comes after the results last month of a Ministry of Agriculture investigation into "the beast phenomena" declared there was no evidence of such a creature.Reuse content