Archaeologists have made an "incredibly exciting" discovery while working on a project to widen a major road - evidence of what is believed to be the first beer brewed in the UK.
The team working along the A14 between Cambridge and Huntingdon said tiny fragments of charred residue from the beer-making process were found in excavated earth.
The experts believe the Iron Age brew could date back as far as 400BC.
"It's a well-known fact that ancient populations used the beer-making process to purify water and create a safe source of hydration," said Dr Steve Sherlock, Highways England archaeology lead for the A14 project.
"But this is potentially the earliest physical evidence of that process taking place in the UK."
Lara Gonzalez - one of about 250 archaeologists on the MOLA Headland Infrastructure project - made the find.
She said: "I knew when I looked at these tiny fragments under the microscope that I had something special.
"The microstructure of these remains had clearly changed through the fermentation process and air bubbles typical of those formed in the boiling and mashing process of brewing.
"It's like looking for a needle in a haystack but, as an archaeobotanist, it's incredibly exciting to identify remains of this significance and to play a part in uncovering the fascinating history of the Cambridgeshire landscape."
She said the porous structures of the fragments were similar to bread but said evidence seen under the microscope of fermentation and larger pieces of cracked grains and bran and no fine flour meant the residue was from the beer-making process.
The former editor of the Campaign for Real Ale's Good Beer Guide, Roger Protz, said East Anglia had always been of great importance to brewing because of the barley grown in the region.
"It's known as maritime barley and is prized throughout the world," the beer expert said. "When the Romans invaded Britain they found the local tribes brewing a type of beer called curmi."
He said the beverage was probably made from grain, adding that hops were not used in the UK until the 15th century.
The archaeologists have already discovered that locals liked to eat porridge and bread while working on the road upgrade project.
They have also discovered a woolly mammoth tusk, several prehistoric burial grounds and an abandoned medieval village.
