A team working on a project to widen the A14 has discovered what is believed to be evidence of the first beer brewed in the UK.
Archaeologists say tiny fragments of charred residue from the beer-making process, potentially from as early as 400BC, were found in excavated earth and suggest an Iron Age brew.
Evidence that locals had a taste for porridge and bread has also been uncovered during work to upgrade the 21-mile stretch of the road between Cambridge and Huntingdon.
Dr Steve Sherlock, Highways England archaeology lead for the A14 project, said: "It's a well-known fact that ancient populations used the beer-making process to purify water and create a safe source of hydration, but this is potentially the earliest physical evidence of that process taking place in the UK."
Lara Gonzalez, who is one of around 250 archaeologists on the project led by MOLA Headland Infrastructure, made the discovery.
"I knew when I looked at these tiny fragments under the microscope that I had something special," she said. "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.
"The porous structures of these fragments are quite similar to bread, but through microscopic study, it’s possible to see that this residue is from the beer-making process as it shows evidence of fermentation and contains larger pieces of cracked grains and bran but no fine flour."
She hopes further analysis into the brewing fermentation process will reveal more.
Beer expert Roger Protz, former editor of the Campaign for Real Ale’s Good Beer Guide, said: "East Anglia has always been of great importance to brewing as a result of the quality of the barley that grows there.
"It's known as maritime barley and is prized throughout the world.
"When the Romans invaded Britain they found the local tribes brewing a type of beer called curmi."
He said this was probably made from grain, and that hops were not used in Britain until the 15th century.
Workers on the A14 project have already uncovered a woolly mammoth tusk, an abandoned medieval village, and several prehistoric burial grounds.
The upgraded road is due to open to traffic by December 2020.