Shops are using new spy techniques to monitor customers' facial expressions, heart rate and even pupil dilation while they browse the aisles.

New technology being installed by retailers in Europe and the US, and being trialed in the UK, can spot if a customer cannot find a product, read their reaction to an outfit or spot thieves in a bid to keep up with online retailers.

Technology installed by one French bookseller is able to detect shoppers' movements and facial expressions and alert staff that they may require assistance.

The anonymous French bookseller claimed its sales rose by 10 per cent after introducing the technology.

Meanwhile stores in Italy have started experimenting with mannequins fitted with intelligent cameras which can detect customers' facial reactions to outfits.

“These systems are designed to be completely invisible so shoppers do not know they are there,” said Mark Thompson, director of retail and hospitality at Zebra Technologies.

“They are mainly designed to improve people's shopping experience but now shops are discovering they can use hi-tech systems to catch shoplifters and learn more about their customers.”