Workers in 10 of 14 food outlets wrongly told a diner who said they had a milk allergy they would be fine to eat a lamb doner.
Later, the kebabs were sent for formal examination and found all to contain milk protein.
Tests also revealed 12 out of 14 kebabs contained other meat species as well as lamb including chicken, turkey, beef and veal.
To support local businesses, West Sussex Trading Standards is advising food outlets on how to improve labelling and allergy information, and is continuing its investigation of offending takeaways to establish more details.
Trading Standards is also backing a major project to ensure food firms know their responsibilities with allergy laws.
The Be Allergy Aware campaign urges businesses across the UK to “double-check, never guess” whether the food they sell contains ingredients that may be allergens, and whether their customers have any food allergies or intolerances.
Richard Sargeant, West Sussex Trading Standards team manager said: “All food establishments – including takeaway outlets - have a responsibility to ensure ingredients in dishes have been checked and implement allergen control procedures. Not doing so is reckless, and highly irresponsible.
"We’re contacting food businesses in West Sussex to make them aware of the results of this investigation and provide them with advice on allergies and descriptions of food."
Duncan Crow, Cabinet Member for Community Support, Fire and Rescue, said: "Trading Standards here in West Sussex will always crack down on offenders who mislead customers.
"Officers rigorously monitor takeaway outlets to detect food fraud and work with businesses to ensure people know what they are paying for."
If you have suffered an allergic reaction following a meal out or takeaway you can let Trading Standards know by calling 0808 223 1133 or go to www.westsussex.gov.uk/tsreport.
Guidance for local food businesses is available on West Sussex Trading Standards website: https://www.westsussex.gov.uk/business-and-consumers/trading-standards-advice-for-businesses.