In its report which surveyed over 1,300 online shoppers found that substituted foods across the grocery industry did not always meet dietary requirements or the customer intolerances.
As a result, 49% of those surveyed revealed shoppers at Aldi, which offers click-and-collect online shopping services only, failed to have their order delivered in full in their most recent shop.
The discount grocer was followed closely by Big 4 grocers Sainsbury’s and Asda, which ranked second and third worst, with 48% and 45% of polled customers reporting at least one replacement item respectively.
Fellow Big 4 grocer, Tesco, found that 39% of customers surveyed had at least one item substituted in their most recent order.
Which? also named Iceland as the best of the nine supermarkets included in the study, with only 18% customers affected on their most recent shop.
On average across all brands, 39% of shoppers reported that their last shop included a replacement item.
Which? retail editor Ele Clark said: “While product substitutions in your online shopping can sometimes be genuinely helpful, our research has shown that they can also be downright ridiculous.
“You do have the right to reject substitutions at the point of delivery, or you could opt out of receiving substitutions altogether – though this can result in a real headache if the key ingredient for your dinner that night is missing.
“If you do end up with a substitution that you don’t want, always contact the supermarket and ask for a refund.”