A quarter of dried oregano sold in British supermarkets and online retailers have been bulked up with other cheaper ingredients, a study has found.
The study for consumer group Which? found that 19 out of 78 samples of the herb contained other ingredients, British newspapers reported.
Between 30 and 70 per cent of of the herbs sold could be made up of other products - most commonly olive and myrtle leaves.
These are almost identical in appearance, and harmless.
Oregano, commonly used in Mediterranean cooking, lend their fragrance to pizzas, grilled meats and vegetables, and salads. It is also used in Filipino and South American cuisines.
The tests were carried out by Professor Chris Elliott, director of the Institute for Global Food Security at Queen's University, who said this finding casts doubt on other herbs and spices sold in Britain.
"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," he said.
Professor Elliott authored a report on food fraud following a massive scandal where horsemeat masqueraded as beef in packaged foods in 2013.
Which? did not name which brands were "fake" but have passed the information to the British food safety agency.
Tests conducted by Which? have found a number of foods to be imposters.
Last year, it found that out of 60 lamb curries and kebabs takeaways bought in Birmingham and London, five had no lamb at all, and 16 were mixed with other meats.