Some 50,000 tonnes of meat supplied by two Dutch trading companies and sold as beef across Europe since January 2011 may have contained horsemeat.
The meat is being recalled where possible, the Dutch authorities say.
There was no evidence that the meat was a threat to human health, the Netherlands Food and Consumer Product Safety Authority said.
In total, 132 companies in the Netherlands and some 370 more around Europe are affected by the discovery.
The suspect meat was supplied by Wiljo Import en Export BV and Vleesgroothandel Willy Selten.
The two companies are owned by one man who has already previously been investigated by food safety officials, the BBC's Matthew Price reports.
The latest find was made as part of EU-wide tests to trace horse DNA in processed beef foods and to detect a veterinary drug used on horses.
Are there no honest people left in the world?