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 additional second in the supermarket equating to an extra 40p at the checkout.
Shoppers in one study added 45 per cent more items to their basket while using a phone, while those taking part in a second study added 58 per cent 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.
“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.”
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 phones did so for an average of 4.82 per cent of their total time in the store. As with the first study, shoppers using mobile phones spent longer in the store.