It came as the company at the centre of the scandal vowed to adopt strict DNA testing of its products to prevent a repeat.
The ABP Food Group, one of Europe's biggest suppliers and processors, is being investigated by health and agriculture authorities in the UK and Ireland after beef burgers supplied by two of its subsidiary companies – Silvercrest Foods in Ireland and Dalepak Hambleton in Yorkshire – to major supermarkets including Tesco, Aldi, Lidl and Iceland were found to contain up to 29% horse meat. A third company, Ireland-based Liffey meats, was also found to be supplying products to supermarkets with traces of horse DNA.
The scandal was uncovered through DNA testing of samples by the Food Safety Authority of Ireland (FSAI), although experts there said there was no reasonable explanation as to how it could have entered the food chain.
NFU Scotland communications director Bob Carruth said consumers should rightly expect that the only meat in a beefburger is beef.
He said: "The undisclosed inclusion of horse meat in some value beefburgers damages the reputation of our food industry and must prompt an immediate investigation of supply chains in place at home and abroad.
"While assurances on the safety of the product have been provided, consumers buying such beefburgers have been seriously misled on the quality and provenance of their food. That is a spectacular own goal for parts of our food sector and doesn't reflect on the fantastic job being done by Scottish beef farmers in providing the market with fresh, tasty, traceable, assured beef."
Liberal Democrat MEP for Scotland George Lyon called for an inquiry into how widespread the practice of meat substitution is.
He said: "Consumers will be astonished that burgers have been found to have contained huge quantities of horse meat in them. There is no way that could have happened by accident. It must have been a deliberate action to substitute expensive beef meat with cheap horse meat."
The FSAI and the UK's Food Standards Agency (FSA) are investigating the source of the horse DNA. Ten million burgers have been taken off shelves as a result of the scandal.
Professor Alan Reilly, head of the FSAI, insisted there is no health risk. He also said FSAI further tests confirmed the meats containing horse DNA did not show any traces of medications frequently given to horses.
However, he added that the presence of horse meat in burgers could not be easily explained.
He said: "Whilst there is a plausible explanation for the presence of pig DNA in these products due to the fact that meat from different animals is processed in the same meat plants, there is no clear explanation at this time for the presence of horse DNA in products emanating from meat plants that do not use horse meat in their production process."
Meanwhile, the FSA is considering whether legal action should be taken against the companies involved and help carry out a UK-wide study of food authenticity in meat products.
ABP Food Group, which is at the centre of the scandal vowed to adopt strict DNA testing of its products to prevent a repeat.
An ABP spokesman said. "It is vital that the integrity of the supply chain is assured and we are committed to restoring consumer confidence."