An ingredient used in Coca-Cola and Pepsi is a cancer risk and should be banned, an influential lobby group has claimed.
The concerns relate to an artificial brown colouring agent that the researchers say could be causing thousands of cancers.
"The caramel colouring used in Coca-Cola, Pepsi, and other foods is contaminated with two cancer-causing chemicals and should be banned," said the Center for Science in the Public Interest (CSPI), a health lobby group based in Washington, DC.
"In contrast to the caramel one might make at home by melting sugar in a saucepan, the artificial brown colouring in colas and some other products is made by reacting sugars with ammonia and sulphites under high pressure and temperatures.
"Chemical reactions result in the formation of two substances known as 2-MI and 4-MI which in government-conducted studies caused lung, liver, or thyroid cancer or leukaemia in laboratory mice or rats."
America's National Toxicology Program says that there is "clear evidence" that both 2-MI and 4-MI are animal carcinogens, and therefore likely to pose a risk to humans. Researchers at the University of California, Davis, found significant levels of 4-MI in five brands of cola.
The executive director of the CSPI, Michael F Jacobson, has petitioned America's food regulator, the Food & Drug Administration, to take action.He said: "Carcinogenic colourings have no place in the food supply, especially considering that their only function is a cosmetic one."
Mr Jacobson said the name "caramel colouring" does not accurately describe the additives, explaining: "It's a concentrated dark brown mixture of chemicals that simply does not occur in nature."
He added that while regular caramel could not be described as healthy, "at least it is not tainted with carcinogens".