Evidence of the first beer believed to have been brewed in the UK, dating back more than 2,000 years, has been uncovered by road workers.

Signs of the iron age brew from about 400BC were identified in fragments of charred residues from the beer-making process found during the £1.5bn upgrade of the A14 in Cambridgeshire.

In parallel with the roadworks, a team of up to 250 archaeologists led by experts, from Mola Headland Infrastructure, a joint venture including Museum of London Archaeology, has been working on the project, investigating 33 sites across 360 hectares, making it one of the UK’s largest archaeological projects.

Lara González Carretero, an archaeobotanist with Mola, said the beer residues were found alongside those of bread and porridge.

“​They look quite similar under a regular microscope, but I was able to do some analysis using a scanning electron microscope [SEM] and there are differences in the insides of the fragments to do with fermentation, which distinguishes them from bread and porridge.

“It’s quite unique. We didn’t have any evidence like this in the UK until now. Beer is very old but we didn’t have physical evidence of it.”

Fragments of charred residue from a beer-making process found at the site. Photograph: Mola Headland/PA

She said all the fragments contained barley, water and oats. “The thing that actually distinguishes [the fragments] is that bread is made of very fine flour. For beer and porridge they are cracked grains. They are bigger. When I looked under the SEM, you could see the starch granules from the beer grains have differences that show fermentation.”

The road project had already yielded a treasure trove of archaeological finds, including whole medieval and Anglo-Saxon villages, 342 burials, dozens of Roman brooches, a bone flute and the remains of a woolly mammoth that could be more than 130,000 years old.

A Roman supply depot, rare Roman coins from the third century, an ornate 8th-century comb made of deer antler and 40 pottery kilns have also been uncovered.

Steve Sherlock, the Highways England archaeology lead for the A14, said the work was continuing to unearth “incredible discoveries that are helping to shape our understanding of how life in Cambridgeshire, and beyond, has developed through history.

“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.”

A set of unusual Roman burials being excavated by archaeologists working on the A14 project. Photograph: Highways England, courtesy of Mola Headland Infrastructure

Roger Protz, lecturer, author of more than 20 books on beer, and the 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 the beer was believed to have been made from grain, as hops did not come into use in Britain until the 15th century, with herbs and spice to balance the sweetness of the malt.

The A14 work has been nominated for the rescue project of the year in the 2019 Current Archaeology awards.