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Using a mobile phone on a trip to the supermarket pushes up shopping bills by 41% on average, a study has found.

The research, by the University of Bath, found shoppers move round stores at a lower pace when they use their phones to keep up with messages, social media or calls.

They also wander along more aisles and come across extra products with each second spent on their phone equating to an estimated extra 20-40p at the checkout.

Shoppers in one study added 45% more items to their basket while using a phone, while those taking part in a second study added 58% more.

This is believed to be because using a phone distracts people from their planned shopping lists and so-called autopilot shop, where consumers take a routine path to their regular items.

Looking at an increased amount of items may jog their memory about things they have run out of, or simply inspire them to make extra purchases.

Dr Carl-Philip Ahlbom, of the University of Bath's School of Management, said: "Retailers have tended to worry that when shoppers use their mobiles it's distracting them from spending money, so we were amazed to find completely the reverse effect.

"The findings were very clear - the more time you spend on your phone, the more money you'll part with.

"So if you're trying to budget, leave your phone in your pocket. It's not the phone itself that causes more purchases, but its impact on our focus.

"On the plus side, it isn't necessarily a bad thing for shoppers.

(Image: PA)

"Taking a slower and more scenic journey can remind you of products you'd forgotten you needed, and it can introduce you to items that might make for a more inspiring menu.

"Shoppers are very habitual creatures, most of us vary our purchases by less than 150 items a year, so maybe you can thank your mobile for freshening things up."

The Stockholm School of Economics, Babson College in Massachusetts and the University of Tennessee also took part in the research.

In the first study, 294 people aged between 18 and 73 at four Swedish supermarkets wore eye-tracking glasses throughout a shopping trip.

These glasses recorded their visual fields and where they fixated their eyes for the time they were in the store.

Researchers measured the amount of time they spent in the shop, the number of times they 'fixated' on a product or price, their movements through the store, whether they used a mobile phone and if so, for how long.

Their receipts were then used to assess their spending.

Results showed that mobile phone use increases the total time spent in the shop and that this results in more purchases.

It also increased the amount of attention that shoppers gave to shelves and diverted them from their usual route.

In this study, shoppers using a mobile phone spent on average 414.40 Swedish Krona (SEK) or £33.73, compared to an average of SEK 293.83 (£23.91) for those who did not.

They purchased an average of 20.61 items, compared to 14.24 and spent 17.39 minutes in contrast to 12.80.

In a second study, 117 shoppers aged between 19 and 80 at two stores were asked to shop as they usually do and wore eye-tracking glasses.

Those who used their mobile phones did so for an average of 4.82% of their total time in the store.

As with the first study, shoppers using mobile phones spent longer in the store, leading to more purchases, and gave more attention to shelves.

This study found shoppers using a mobile phone spent on average SEK 444.28 (£36.16) on 20.85 items, compared to SEK 314.37 (£25.59) on 13.22 products bought by those not using one.

Those using a phone were in the store for an average of 15.37 minutes, while those who did not spent 10.92 minutes inside.

Researchers believe this is because phone use interrupts the "autopilot" that people's brains activate to process the many thousands of products in supermarkets.

Professor Jens Nordfalt, of the University of Bath, said: "For retailers there's a clear message here that they no longer need to fear mobile phone use in-store.

"In fact, making it easy for customers to use their mobiles, with good WiFi and enhancements like mobile phone docking stations on shopping trolleys, will more than pay off.

"The one exception is that using a mobile phone protects shoppers from temptation at the checkout. Here we found that people picked up fewer items than normal."

Researchers believe the findings would be similar in other types of shopping experience, such as low or moderately priced fashion stores.