Fried food may contain the chemicals
The UK Food Standards Agency commissioned urgent checks after Swedish researchers found chemicals called acrylamides in food.
Their findings - although limited to potatoes, crisps and breakfast cereals - confirm these results.
They suggest that baking or frying the food may be to blame.
This means that humans may well have been exposed to these chemicals for generations.
Tough limits
Acrylamides are normally found in certain packaging materials.
However, the FSA testers found that in fried potato chips and crisps, the level was far in excess of this.
In normally-fried chips, there were more than 300 parts per billions - while in overcooked chips this rose to more than 12,000.
Several brands of crisp had more than 1,000 parts per billion, while one toasted breakfast cereal contained more than 300.
The chemical was not found in uncooked or boiled potatoes.
Gene damage
Acrylamide came to the attention of scientists after experiments on cells in the laboratory, and on animals suggested it could be harmful.
It caused damage to the genetic structure of cells - smoking is another way a human could be exposed to the chemical.
However, it is not known what is a safe level of exposure.
Dr Andrew Wadge, from the FSA, said: "We are all exposed to natural chemicals that make up the food we eat.
"Obviously we want to do everything we can to reduce or remove potentially harmful substances from food.
"However, at this stage it is too early to identify either the effects of acrylamide in food on people or even how it is formed in processes such as baking, frying, grilling or roasting."
He added: "It is likely that any risks from acrylamide are not new and we have probably been exposed to them in food for generations."
He said that future studies might help scientists work out the exact risk - if any - to humans.
And he said that people should not change their diets in response to the news.