Dunnes Stores engaged in a misleading commercial practice likely to deceive customers in an advertising campaign comparing some products with rival Aldi, the High Court ruled.
Dunnes infringed consumer protection law and EU regulations, Mr Justice Brian Cregan ruled. He was giving judgment in an application by Aldi for an injunction preventing Dunnes infringing its trademarks.
Aldi is also seeking damages - while Dunnes is to consider whether to seek a stay on the judge's decision in the event of an appeal.
Aldi had claimed that in 2013 Dunnes infringed its trademarks through the use of shelf labelling and free-standing shop floor advertising "banners" claiming cheaper prices.
The advertisements, used in Dunnes' outlets around the country, stated: "Lower Price Guarantee" and "Guaranteed Lower Prices on all your Family Essentials every week".
It was claimed the advertising failed to comply with the Consumer Protection Act 2007 and the European Communities (Misleading and Comparative Advertising) Regulations 2007 on grounds including they failed to objectively compare one or more of the relevant features of the Dunnes products with those of Aldi Ireland.
The banners also conveyed the impression Dunnes products generally, or its "Family Essentials" range, were cheaper than those of Aldi Ireland when there was "no basis" for such a claim, Aldi said.
Dunnes denied the claims and said the advertisements were lawful.
Mr Justice Cregan found that in relation to pricing labels on Dunnes' shelves on 14 out of 15 products, the company had infringed the regulations and Irish law.
The products included pork sausages, turkey breast mince, yogurts, day cream, shower gel, toilet tissue, a sparkling orange drink, tomato ketchup, white sauce, tinned beef and chicken dog foods and dry cat food.
Mr Justice Cregan found "shelf edge labels", claiming Dunnes prices for these products were lower than Aldi prices, did not objectively compare products meeting the same needs or intended for the same purpose.
He also said the advertisements included "the provision of false information" in relation to the 14 products, including information about the nature, composition, characteristics or ingredients.
He concluded such information would be likely to cause the average consumer to make a transactional decision, which that consumer would not otherwise make.
The advertisements were misleading because they were likely to cause the average consumer to be deceived or misled in relation to those 14 products.
He noted other issues including that the Aldi sausages had a Bord Bia approval logo, the Aldi toilet tissue was heavier and longer, while a yogurt contained 13pc more strawberry that the Dunnes product.
Commenting on the case, Aldi's Group Buying Director, Finbar McCarthy, said that the ruling benefits consumers.