The consumer watchdog said 19 of 78 samples contained other ingredients, most commonly olive and myrtle leaves but also cistus.
These were found to make up 30% to as much as 70% of the product.
Samples were bought from shops in the UK and Ireland and from online retailers in spring this year.
Which? said it was unable to give any further detail on the testing as the snapshot research was only intended to highlight the issue in the market.
It remains unclear where the samples tested originated from, whether the deception was intentional or not and potential reasons behind it.
Seasoning and Spice Association response
The Seasoning and Spice Association (SSA) said to ensure quality and integrity of products, all companies have controls in place to protect themselves and customers from fraud.
“The majority of oregano sold in the UK is authentic, as this survey indicates. That some samples have been found to contain large quantities of bulking substances instead of oregano is unacceptable and undermines the efforts of many suppliers to ensure the integrity of this supply chain.
“We are working with the FSA and retailers to ensure that the most up to date knowledge and good manufacturing practices are shared more widely to ensure more companies can identify and manage the risk of food fraud more effectively.”
The tests, done by mass spectrometry, were conducted by Professor Chris Elliott, director of the Institute for Global Food Security at Queen’s University in Belfast.
Mass spectrometry was used in Canada to reverse initial findings of cumin products containing almond in the nuts and spices issue, finding the readings were actually caused by mahaleb.
Survey ‘based on intelligence’
Professor Elliot said based on intelligence received it decided to see if there were issues with the authenticity of oregano supplied in the UK and Ireland.
“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 results were shared with the Food Standards Agency (FSA) who said it is looking into the issue but was not putting a timeline on any investigation.
The agency said it is the responsibility of food suppliers to test products to ensure consumers are not being misled.
“The potential for food fraud in the herbs and spices sector is something that the FSA is already exploring following concerns about undeclared allergens in spices earlier in the year.
“We will continue to work with the UK food industry to ensure products are being appropriately tested and correctly labelled. The results from the Which? survey will be followed up with the UK food businesses concerned.”
Richard Lloyd, Which? executive director, 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.”