A genetically modified crop eaten by millions of people and animals around the world may be unsafe because of previously undetected toxic effects, a study has claimed.

Maize that has been modified to survive being sprayed with high doses of weedkiller contains “worryingly large increases” in substances that can worsen allergic reactions and increase the risk of cancer, according to research by King’s College London.

The NK603 maize has been widely planted in the US, Canada, Brazil and Argentina since being approved more than a decade ago. Monsanto, which produced it, said that it was “substantially equivalent” to non-GM maize. It is popular with the farming industry because it is resistant to glyphosate, the world’s most widely used weedkiller.

It is eaten by humans around the world but in Europe is usually only consumed by animals because of restrictions on GM crops.

The researchers said they analysed the molecular composition of the maize in far greater detail than previous industry-led studies.

The plant’s modification altered the protein content of maize kernels and exposed cells and tissues to greater damage when reacting with oxygen, the researchers found. The NK603 maize also contained increased levels of cadaverine and putrescine, which are thought to be involved in the formation of carcinogenic substances. They can also heighten allergic reactions by enhancing the effects of histamine.

Michael Antoniou, lead author of the study in the journal Nature Scientific Reports, said: “Our results call for a more thorough evaluation of the safety of NK603 [maize] consumption on a long-term basis.”

Peter Melchett, policy director of the Soil Association, the organic certification body, said: “It may start to explain why farmers have found that some GM crops, which are claimed to be the same as non-GM, have caused health problems when fed to their animals.”

Joe Perry, former chairman of the European Food Safety Authority’s GM panel, said the study would need to be replicated to ensure that differences in the fields used by the researchers to grow the GM and non-GM maize did not affect the results.