Tesco says it will have more in-store self shopping scanners than any other UK retailer following a move this week to extend the service across its estate.

The UK's largest retailer announced today that it is rolling out its Scan as you Shop service to a further 20 stores, accompanied by a TV advert to promote the technology. The supermarket says the move is a bid to help customers save time at the checkout.

The Scan as You Shop service allows customers to scan products as they walk round the store, and keep a tally of how much they are spending. The system will be available for use at self-service checkouts as well as manned checkouts, and the service is now available in nearly 350 stores across the UK.

A Tesco spokesperson said the service is already used by more than 600,000 of its customers every week, adding: "We want to make it as easy as possible for customers to shop with us.

"This new trial will mean more customers than ever before can benefit from the service and check out in a matter of seconds."

Sainsbury's and Waitrose are among the other supermarkets that offer customers the chance to use self-scan machines in their stores, and there are various trials going ahead to combine this technology with consumers' own mobile devices.

Retailers are being encouraged to offer their customers choice and variety when it comes to the way they can embark on their shopping journeys, and scan-as-you-go technology meets that need. However, a recent report by University of Leicester criminologists said that mobile-scan-and-pay tech is promoting theft.

The report suggested that mobile-scan-and-pay technologies generate a significantly high rate of loss for retailers, as the technology removes human contact from the shopping process. It was said that the technology makes it easier for shoppers to steal, while also reducing the perception of legal consequence.

Dr Matt Hopkins, from University of Leicester's Department of Criminology, commented: "Retailers are becoming aware of these problems and introducing ways of amplifying risk in the mobile scan and pay environment, trying to ensure that all that ends up in the basket also makes it onto the receipt."