Testing has revealed a quarter of dried oregano bought from a range of retailers contained other ingredients.
The research, carried out by the Institute for Global Food Security and released by Which?, found the most common substitute ingredients were olive and myrtle leaves.
Professor Chris Elliott, director of the institute, said: “Clearly we have identified a major problem and it may well reflect issues with other herbs and spices that enter the British Isles through complex supply chains.
“Much better controls are needed to protect the consumer from purchasing heavily contaminated products.”
The testing, which used mass spectrometry to identify compounds by their atomic composition, involved 78 samples from shops in the UK and Ireland and online stores.
Olive and myrtle leaves were found to make up between 30 per cent and 70 per cent of certain products.
Which? executive director Richard Lloyd said: "It's impossible for any shopper to tell, without the help of scientists, what herbs they're actually buying. Retailers, producers and enforcement officers must step up checks to stamp out food fraud.”