Shoppers spend 17.22 or 26 per cent of the average 65 weekly grocery bill on food such as chocolate, alcohol, biscuits, crisps and soft drinks, according to the study for weight loss company LighterLife.
The tally did not include mid-week shops, petrol station purchases, coffee runs or lunchtime impulse buys.
The survey found Asda shoppers spent the most on unhealthy food at 18.23, while Tesco customers were the healthiest, spending 16.65 on junk items.
More than a third (37 per cent) said they always overspent at the supermarket because they were tempted by treats, while 39 per cent said they expected to overspend on treats.
The poll showed 81 per cent of people bought extra snacks outside of the supermarket shop.