A Daily Telegraph investigation found pork in Sainsbury’s own-label Meat Free Meatballs, and traces of turkey in Tesco’s vegan Wicked Kitchen BBQ Butternut Mac ready meal.
An FSA spokeswoman said: “Our priority is to ensure consumers can be confident that the food they eat is safe and is what it says it is.
“We are investigating the circumstances surrounding these alleged incidents and any resulting action will depend upon the evidence found.”
Both retailers are also carrying out investigations into the findings.
A Sainsbury’s spokeswoman said: “These products are produced at a meat-free factory.
“Sainsbury’s and the Vegetarian Society also carry out regular checks and no issues have been found.
“We are concerned by these findings however and are carrying out a comprehensive investigation alongside our supplier.”
Lynne Elliot, chief executive of the Vegetarian Society, said:
“We have stringent criteria that any product must meet in order to display one of our trademarks. We check ingredients and suggest alternatives where necessary, check production processes including possible cross-contamination points, offer advice on best practice, and assess risk. We conduct site visits where needed but, for example, where a facility manufactures exclusively vegetarian products, this would usually be considered unnecessary.
“It is the producers’ responsibility to ensure production processes are followed and to reduce as far as possible the chance of human error. In the unlikely event of a mistake happening, we are always happy to work with a producer to help minimise the chance of future occurrences.
“We have found that every company we have ever worked with wants to do their absolute best for their vegetarian or vegan customers and we are proud to work with such an extensive range of companies, big and small.”
A Tesco spokesman said the retailer had tested the product following the Telegraph’s investigation.
“We take the quality and integrity of our products extremely seriously and understand that our vegan and vegetarian products should be exactly that,” he said. “Our initial DNA tests have found no traces of animal DNA in the BBQ Butternut Mac product available in stores today.
“We would urge the Telegraph to share full details of their testing, including the lab used, as we continue to investigate.”