EU Justice, Consumer Protection and Gender Equality Commissioner Vera Jourova said the European Commission had issued a legal opinion that such quality differences were unfair and the problem "could be resolved very soon".
The practice is legal in the EU as long as food ingredients are clearly listed.
Officials in ex-communist Poland, the Czech Republic, Slovakia, Hungary and Bulgaria, however, have said it is unethical for food items sold under the same brand to be inferior in quality in "new" EU member states in the east compared with "old" member states in the west.
"Double standards in food products in the European Union have left 100 million people (in the EU's east) feeling like aborigines," Bulgarian Prime Minister Boyko Borissov told a conference on consumer law and consumer protection in Sofia.
"I would be very ashamed if I was in place of these chains and manufacturers. Does it not seem a shame? Who is this clever guy in Europe who decided a Bulgarian baby prefers palm oil to milk?"
Borissov, whose Balkan nation holds the EU's rotating presidency, said he would ensure food from Bulgarian supermarkets was served to EU leaders at a forthcoming EU-Western Balkans summit in Sofia next month.
"EU unity means one standard," he said.
Three weeks ago the European Commission proposed a so-called New Deal for Consumers entailing directives against unfair business-to-customer commercial practices as well as action to protect collective consumer interests. The proposals will be discussed by the European Parliament and the EU Council.
"We are introducing harmonisation of penalties that should be imposed in cases of cross-border infringements," Jourova told the conference, with new legislation expected to be passed by the bloc's parliament in the spring of 2019.
"There must be no second-class consumers in the EU, it is not fair," she said.
Last week a study found differences in ingredients and prices of some brands of food products between those sold in Bulgaria and in four western EU countries, Sofia's Agriculture Ministry said.
Consumer groups in Bulgaria have repeatedly complained about the use of different ingredients in identically branded goods.
Lab tests have shown that some multinational brands use different, often cheaper ingredients in food sold on the east side of Europe's old Iron Curtain divide than in products sold in adjacent Austria and Germany.
Companies have explained differing composition of products sold in less competitive, poorer eastern markets by saying they are catering to local tastes and preferences.
(Reporting by Angel Krasimirov; Editing by Mark Heinrich)
