Dried oregano is the focus of "the latest in a long line of food frauds", with a quarter of samples containing other ingredients, suggests a study.
Of 78 samples of the herb, 19 contained ingredients like olive or myrtle leaves, researchers found.
The study, for consumer group Which? revealed that in some cases, less than a third was oregano.
"Much better controls are needed," said lead author Prof Chris Elliott of the Global Institute of Food Security.
Prof Elliott examined dried oregano sold at a range of shops in the UK and Ireland and from online retailers.
The team used a technique called mass spectrometry to identify the make-up of the samples.
Some contained between 30% and 70% of other ingredients.
"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," said Prof Elliott.
Consumers need much better protection "from heavily contaminated products," he added.
Prof Elliott blamed the complexity of food supply systems for the problem and suggested a lot of food may not be what consumers think it is.
"It particularly happens with things that come from far away places and with many different people interacting in the supply chains, they do tend to be very, very vulnerable," he told BBC Radio 5 live's Breakfast programme.
In a government-commissioned report, published in the wake of the 2013 horse meat scandal, Prof Elliott called for a national food crime prevention network to help protect consumers from food fraud.
In separate work last year in conjunction with Which? he found 40% of lamb takeaway meals contained other meat and one in six of the fish it bought from chip shops was not what had been ordered
Which? described the findings as "the latest in a long line of food frauds" which it has revealed and has shared them with the Food Standards Agency.
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."
The Food Standards Agency said it would follow up the results of the Which? survey with the UK food businesses concerned, adding that an investigation into the herbs and spices sector was already underway.
"It is vital that the food people buy is what it says it is on the label," said a spokeswoman.
"It is the responsibility of food suppliers to test their 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."