Carrots are one of the cheapest vegetables by weight, which has led to suspicions that supermarket customers are abusing electronic checkouts in a shoplifting scam.
Emmeline Taylor, a senior lecturer in criminology at City, University of London, said people switch labels or deliberately input the wrong item to pay less for produce.
She told The Times newspaper that she first spotted the trend in Australia and it is also happening in Britain.
"I was working with retailers to reduce shoplifting when one major supermarket discovered it had sold more carrots than it had ever had in stock," she said.
"Puzzled by this development it looked into its inventories and found that in some cases customers were apparently purchasing 18kg of carrots in one go.
"Unfortunately this wasn't a sudden switch to healthy eating, it was an early sign of a new type of shoplifter."
She said that product switching has become so common in the UK that some people doing it had forgotten that they were committing a crime.
"This behaviour is perceived as cheating the system or a way of 'gamifying' an otherwise mundane routine," she added.
More than Â£3 billion of goods are estimated to be stolen through Britain's 50,000 self-service tills each year, The Times reported.
Theft from unmanned checkouts has more than doubled over the past four years, according to reports.