Supermarket “pricing secrets” mean that customers are paying up to four times more for groceries depending on where and when they shop, according to Which?.
The consumer group analysed 493 branded items sold in Tesco, Sainsbury’s, Morrisons, Asda, Waitrose and Ocado throughout 2020.
Ocado’s Lavazza Qualita Rossa Ground Coffee (250g) saw the most dramatic price difference, fluctuating between £1.30 and £5.
The online grocer said the difference was an error that has now been corrected.
Which? warned customers against “high-low” tactics, where a supermarket raises prices then slashes them at regular intervals.
For example, Müller Light Greek Luscious Lemon yogurts at Sainsbury’s fluctuated between £1 and £2.75 at roughly three-week intervals.
Similarly, a bottle of Shloer’s Red Grape Juice Drink at Morrisons regularly changed by 125 per cent.
Cake and biscuits proved the most volatile category, varying by around 48 per cent on average.
The biggest price difference in this category was an Asda 10-pack of Cadbury Chocolate Mini Rolls, which cost £1.20 on some days and £2.60 on others.
However, Asda was found to have the lowest overall grocery prices, while Waitrose and Ocado had the highest.
“Our research reveals just how wildly food and drink prices can fluctuate from day to day, meaning people are at risk of massively overpaying for branded groceries,” Which? retail editor Ele Clarke said.
“We would recommend keeping an eye on the prices of your favourite products and stocking up when they’re discounted to avoid paying over the odds.”
The research did not include Aldi or Lidl as they stock fewer branded goods.