The so-called “swipers” swap barcodes, price up fruit and veg as cheaper produce and use other tricks at unmanned checkouts at an annual cost to retailers of £14 million.

“Often they don’t see it as stealing because they’re interacting with a machine,” said a criminologist.

In a recent survey a quarter of people admitted to stealing via self-scan checkouts, the equivalent of 1.1 million Scots.

A quarter of self-scan customers admit theyve stolen groceries from supermarkets in Scotland. Picture: Photofusion/REX/Shutterstock

According to retail industry figures, stores with self-scan facilities lost 3.97 per cent of stock compared to just 1.47 per cent if they have traditional staffed tills.

Retailers admit the costs of these thefts are being passed on to ordinary consumers, but money saved on staff means the number of unmanned scanners will continue to increase. At airports, automated check-in machines are said to cost airlines 10p per transaction against £2.50 to check someone in via a staff-member.

David Lonsdale, of the Scottish Retail Consortium: “Consumers increasingly want their shopping to be quick and convenient, and the roll-out of self-scanning machines is being driven by that consumer behaviour and also the fact that the cost and capability of technology is becoming more affordable and attractive.

“Unfortunately there are some bad apples determined to steal. However, our members are working hard to guard against criminality, working with their suppliers to build in appropriate security systems and liaising with law enforcement.”

Crime prevention measures including special “mirror” screens, facial recognition technology and 3D sensors are being introduced to keep shoppers honest.

Dave Williams, of the Union of Shop, Distributive and Allied Workers, said: “We suggest that retailers think very carefully about the knock-on impact to their business of reducing staffing levels through the introduction of new technology.”

The first self-service checkouts were introduced in the 1990s and in Britain alone the number of these tills grew from 7,000 in 2008 to 42,000 in 2015. Scots thieves are estimated to steal goods worth an average of £350 annually. Dr Emmeline Taylor, a criminologist at City University, London, said: “Somewhere between one in three to one in five admit to stealing via self-scan checkouts.

“Many people begin by doing it accidentally, but then they see how easy it is and then it becomes a regular habit. Often they don’t see it as stealing because they’re interacting with a machine.”

One in four Scottish shoppers are stealing from self-serve machines