South African health authorities have stepped up random tests on meat and other foods in supermarkets after a study showed that donkey, goat and water buffalo have been added as unlabelled ingredients in meat products.

The investigation started last month in response to a scandal in Europe where horse meat was labeled as beef, health department spokesman Popo Maja said.

It took on more urgency after University of Stellenbosch researchers said on Tuesday unconventional and unlabelled ingredients were found in nearly two thirds of South African hamburgers and sausages tested.

"Investigations in the form of random tests are under way to ensure everybody complies with the food labeling regulations," Maja told Reuters on Wednesday.

Shares in major South African retailers fell in late afternoon trade on worries that an industry-wide investigation could trigger product recalls.

"Retailers have been under pressure recently because of valuation issues but the meat scandal is the reason for today's sell-off," said Paul Chakaduka, a trader at Johannesburg-based Global Trader.

Major food retailers Shoprite , Pick n Pay and Woolworths said they believed all of the meat for sale at their stores was properly labeled.

"We do not believe that our meat products are implicated in this study," said Woolworths, which is similar in style and products to Britain's Marks & Spencer Plc.

Shoprite, which imports some pork cuts and oxtail from Germany, Argentina and Australia, said it would penalize any supplier should any of their products be implicated in the university's study.

"We do not believe that any of our suppliers, who are reputable companies, would transgress food standards and labeling regulations," said Whitey Basson, the company's chief executive officer.

The European horse meat scandal led to recalls of ready meals and damaged confidence in the continent's complex food industry. It also raised concerns worldwide over the risks to human health from a complex and nebulous meat supply chain.

(Reporting by Tiisetso Motsoeneng; Editing by Jon Herskovitz and Mark Heinrich)