Retailers are misleading shoppers into buying own-brand products which "borrow" elements from the packaging of well-known competitors, according to a watchdog.
A fifth of Which? members said they had accidentally bought a supermarket version of a favourite brand at least once with 60% of those saying the mistake left them feeling annoyed or misled.
The consumer group found more than 150 own-label products had mimicked the packaging of products such as McVitie's digestives, Kellogg's coco pops, Simple cleanser and wipes, Radox bath gel and Jacob's cream crackers.
Lurpak butter seemed to have "a recognisable own-label imitator" in most major supermarkets, Which? said.
Own-label products, which tend to be cheaper than brands, are becoming more popular among shoppers struggling with tightened finances and rising food prices, according to separate research from the group.
Its survey on own-brand packaging found 18% of members had deliberately bought an own-label product because it resembled the branded equivalent, with 60% of these shoppers doing so because it was cheaper and 59% wanting to see if it was as good.
But consumers looked upon own-brand products less favourably when they were confused by the packaging, with 38% of those who bought such a product by mistake saying it annoyed them and 30% reporting that they felt misled.
British Brands Group director John Noble said: "Our research shows that consumers are more likely to buy own-label products if they look like brands.
"Brands survive by being distinctive and standing out, and retailers are free-riding on brands' reputations.
"Currently in the UK there is little to stop a competitor packaging its product to look like a familiar brand, whether or not the product's performance is in any way similar.
"That can't be good if we want a market in which shoppers can make informed decisions at speed."
Boots said that colours could be synonymous with certain active ingredients and helped consumers find the right product, while Morrisons, Superdrug and Aldi all said retailers used the same colours as branded products to help customers find products quickly.
A Which? spokeswoman said: "Own-brand products can provide good value and several have topped our tests to become best buys.
"But retailers should make sure that people are under no illusions about what they are buying and not leave so many consumers feeling that they have been misled."