Sainsbury is labelling its prepacked onions as Class 1 from next week, after talks with DEFRA.
Historically, the trade has always graded the product Class II to ensure it complies with EU grading regulations.
First in the range are red New Zealand onions sold in its Premium brand, brown onions from the same source and onions known as three in line nets from Chile.
Russell Crowe, Sainsbury's senior buyer, said: "We are not raising standards but are able to upgrade the crop because of the strength of our partnership with growers.
"We will also ensure customers recognise the difference."