Scientists at Nottingham Trent University (NTU) tested 57 products from 10 unspecified retailers and found that seven contained undeclared offal and five contained blood serum.
The results come a year after the horse-meat scandal which led to products such as burgers and lasagne being removed from supermarkets after they were found to contain traces of horse.
According to NTU scientist Professor Ellen Billett, the undeclared offal was detected at a level of 1 per cent or more, with some products containing more than one type of offal. The report said the five products with undeclared blood contained serum "at a much higher level than expected".
The Food Standards Agency (FSA) and the Department for Environment, Food and Rural Affairs (Defra) said it was working with NTU to develop validated testing methods. The results of this work are due to be published in late summer.
A Defra spokeswoman said it was not possible to draw conclusions from the study until there was "a robust method that is technically sound" in place.
An FSA spokesman said local authorities would follow up on any undeclared offal found in products.