Burger King has reintroduced some of the food colours associated with possible hyperactivity in children in a small number of restaurants, the Food Standards Agency (FSA) said.
The fast-food chain is using the colours as a result of trialling the Coca-Cola Freestyle drinks dispensing system for six months until December.
The FSA said it had removed Burger King from its list of caterers and restaurants whose food and other products were free of the colours.
The vending machine allows customers to choose from a variety of flavours, but Coca-Cola had been unable to remove all of the colours associated with possible hyperactivity from the system before the trial, the FSA said.
It added that Coca-Cola had "reaffirmed its commitment" to remove the colours from its entire product range.
An FSA-funded study identified six food colours that have been associated with possible childhood hyperactivity: sunset yellow FCF (E110), quinoline yellow (E104), carmoisine (E122), allura red (E129), tartrazine (E102) and ponceau 4R (E124).
It is encouraging the food industry to voluntarily remove the colours from their products and publish their details online as they do so.
Coca-Cola said the presence of the so-called Southampton Colours, named after the study which identified the behavioural link, were well below the "legally required acceptable daily intake".
A spokeswoman said: "Coca-Cola beverages and the ingredients in them are absolutely safe and comply with all governmental requirements in more than 200 countries where they are sold.
"Coca-Cola Great Britain last month started a six-month trial of Coca-Cola Freestyle, the company's innovative fountain dispenser, in 16 Burger King restaurants in greater London.
"This is a limited, small-scale trial based on ingredient combinations that have been used successfully in other markets.
"The presence of Southampton Colours in the Coca-Cola Freestyle drinks involved in this trial is well below the legally required ADI (Acceptable Daily Intake). To put this into context, an individual would need to consume multiple litres of product in one day to exceed their ADI for a Southampton Colour.
"As this is a limited, small-scale trial it is too early to say whether we will launch Coca-Cola Freestyle on a permanent basis.
"In response to changing consumer preferences, we have been moving towards the use of non-artificial colours where viable alternatives can be found and this will be the case if the Coca-Cola Freestyle trial is rolled out nationwide in future.
"Both the Coca-Cola Company and Burger King UK Ltd are committed to helping people make informed choices about the products we both offer.
"We have worked closely with Burger King UK Ltd to ensure that all nutritional and ingredient information relating to Coca-Cola Freestyle is readily available in the 16 restaurants participating in the trial."