Aldi and Lidl claim the food and drink sold in their stores is cheaper but as good as that sold in Marks & Spencer and Waitrose.
In their submissions to the Competition Commission's groceries inquiry, the hard discounters insisted it was simply the image of rival stores, including the big four, that created the perception that their offers were of a higher quality.
"In quality terms, there is no difference if individual products are compared on a direct basis," said Lidl.
"Customers' quality perception has got a lot to do with the perceived image of the respective retailer. Many of Lidl's and Aldi's products are of the same quality as Marks & Spencer's and Waitrose's but customer perception still widely disacknowledges this matter, although a look at the ingredients lists on the packaging would easily provide enlightenment."
Aldi compiled a table for its submission in which it categorised all major grocery chains and compared them to its own business.
The big four, Waitrose, M&S and Booths all offered bigger ranges than Aldi stores, it conceded, but also charged "higher prices for like-for-like quality".
It claimed other rivals - including Lidl - offered "poorer value for money" and a "poorer store environment".